Understanding the basics of motor oil, your motorhome’s most vital fluid.
By Peter D. duPre
Most people think motor oil exists simply to lubricate internal engine parts and keep them from failing prematurely due to excessive wear brought on by friction and internal combustion. Lubrication, of course, is the primary job of motor oil. Preventing wear and scuffing on internal engine surfaces by coating them with a clingy yet thin protective layer of oil is a vital process of which we are all aware. While most of us have never run an engine out of oil, we all know that without lubrication, a running engine quickly becomes a ruined engine.
In the extreme operating environment of the internal combustion engine, finely machined metal parts slide or rub directly against each other, causing friction that in turn creates heat. To the naked eye, pistons, cylinders, bearings, and other engine parts look extremely smooth, but a microscopic inspection of even brand-new engine parts would reveal surface irregularities that look as large as canyons. If two engine surfaces rubbed directly against each other, such as a piston ring and a cylinder wall, without the thin protective layer of lubricating oil, the irregularities in the surface of the metals would grind against each other like sandpaper does on wood. Small particles of metal would be scoured from the metal surfaces, and these would soon gouge out larger particles. In addition, the dry friction of the two metal surfaces rubbing against each other would create enough heat to literally melt metal. That’s why dry engines become solid blocks of steel that are suitable only for anchoring a mooring buoy in some remote Alaskan harbor.
Add a protective layer of oil between these parts, however, and finely machined engine parts move smoothly and don’t seize together. The imperfections in the metal surfaces are filled in with a thin layer of oil that adheres to the two surfaces, while the layer of oil between the two surfaces actually slips a little as the parts move. This is called viscous friction, and the ability of oil to flow under these conditions is called viscosity. Simply put, oil with low viscosity flows more easily, so the viscous friction between the surfaces is fairly low. Oil with a higher viscosity flows more slowly, and the resulting viscous friction is higher. So 30-weight oil will flow more quickly than 50-weight oil.
Oil’s Other Functions
What many people don’t realize is that apart from lubrication, oil performs a number of other important functions, including cleaning, cooling, sealing, and shock absorption.
Cleaning. The inside of an operating engine is a dirty place. As the engine runs, soot, ash, acids, unburned fuel, and moisture are created by the process of fuel combustion, which can form varnish, resins, and sludge. These substances can become baked onto engine parts, clog passageways, and interfere with overall performance. The oil circulating throughout the engine carries away such impurities, keeping them suspended in the fluid until they pass through the pan (where larger particles settle) and the filter (where smaller particles and impurities are trapped).
Cooling. Only about 60 percent of engine cooling is achieved by coolant mixture transferring heat through the radiator fins and into the air. The other 40 percent is cooled by motor oil, particularly in the lower parts of the engine. As oil circulates throughout the engine, it draws away heat generated by combustion and friction. The hot oil passes this heat off through an oil cooler and in the pan, where some air passes around it, which further lowers its temperature.
Sealing. We’ve already seen that a thin coating of oil fills in or seals the slight imperfections on the metal surfaces, making them smooth and less receptive to friction damage, but oil also acts as a seal between the piston rings and cylinder walls. In order to glean the most power and efficiency from the combustion act, there needs to be a seal between the edge of the piston and the cylinder wall. The compression rings on each piston do most of this job, but because of the nature of moving parts, they cannot completely seal the space between the piston and the cylinder wall. The motor oil not only lubricates the rings so they don’t stick to the piston, but it also provides viscous friction with the cylinder walls and piston ring grooves, creating a complete seal and further reducing blow-by of combustive material past the rings.
Shock absorption. The forces on engine internals during the combustion process are tremendous. When the compressed fuel/air mixture ignites in the combustion chamber, the pressure in the chamber increases exponentially. A load measuring as much as three tons is exerted on the top of the piston and is then transferred down through the piston, piston bearings, wrist pin, connecting rod, rod bearings, and crank journals. Because oil fills the voids between these items, it acts as a cushion, absorbing enough of the load to keep shock damage from occurring.
In addition to the above, engine oil has to be thin enough to let the starter turn over the engine (just try filling your engine with straight 40-weight and turning the key during a North Dakota winter!). It also needs to be able to circulate rapidly throughout the engine, yet be thick enough to provide complete engine protection without performance loss under a wide temperature range (ambient temperature to more than 225 degrees Fahrenheit). Generally, the higher the range that oil can achieve in staying thin at colder temperatures while maintaining thickness at higher temperatures, the better its quality. Engineers call this range the viscosity index and use it as a measure of the oil’s ability to respond desirably at temperature extremes.
Whether conventional or synthetic, all motor oils contain additives that help the oil meet the above performance requirements, improve the oil’s overall performance, and help reduce wear and tear. Some of the more common additives include:
Anti-wear agents. A zinc-phosphorus compound is most commonly added to provide a protective film on moving parts that helps to further reduce friction and to limit engine wear, but other anti-wear agents may be added along with this compound or may replace it entirely.
Detergents. The purpose of adding detergents to the oil is to help its cleaning ability. Just as laundry soap cleans your clothes, oil detergents loosen carbon deposits, gum, dirt, and varnish from metal parts.
Dispersants. These chemicals help to keep particles in oil from clumping together and in suspension so they can be filtered out or easily drained away at oil change.
Foam inhibitors. As air moves through the crankcase, it can churn up bubbles (foam) in the oil that can retard proper lubrication. Foam inhibitors reduce foam buildup and retention by causing bubbles to rapidly disperse and break.
Friction modifiers. These additives reduce friction between moving parts, to reduce low-temperature wear during startup and also to enhance fuel economy/efficiency.
Oxidation inhibitors. The high operating temperatures and the natural agitation of the oil as it circulates throughout the engine mean that a certain amount of oxygen gets mixed into the oil. This oxidizes the oil, causing it to break down and form sludge. Oxidation inhibitors reduce the amount of oxidation and the formation of sludge.
Pour point depressant. The purpose of this additive is to prevent wax crystallization in conventional oils at extremely low temperatures (below 0 degrees Fahrenheit), so that engine cranking and oil flow are improved.
Rust and corrosion inhibitors. These counteract rust, wear, and corrosion by displacing water from metal surfaces and neutralizing acids occurring from the combustion process.
Seal swell agent. Acids and chemicals in the oil tend to make seals and gaskets shrink over time. Special chemicals are added to the oil to cause a slight swelling of gaskets and seals, helping to prevent oil leaks.
Viscosity index improvers. As motor oil heats up during engine operation, it has a natural tendency to expand or thin out, reducing viscosity, and as it cools it tends to thicken. These chemicals help the oil to resist viscosity change as operating temperatures fluctuate.
Although many of these additives may be present in all oils, the specialized engine type/usage often requires different formulations or mixes of oils and additives. Lawn mowers, ATVs, motorcycles, race cars, compact cars, sedans, SUVs, buses, RVs, heavy trucks, boats, earthmovers, and other machines all are powered by different sizes and types of engines (two-stroke gasoline, four-stroke gasoline, diesel, and even hybrid). Each has its own unique set of operating parameters, and oil manufacturers consider all of them when formulating specific oils. So, while all oils have additives, they don’t all have the same additives or the same amount of additives, nor even the same viscosity grade. Thus, it is important to always make sure you are using the correct oil for your engine, and, because the additives are used up over time, to regularly schedule oil and filter changes.
One final word on additives: Using an over-the-counter oil additive with your existing motor oil is not a good idea. Modern motor oil is specifically engineered for an exact set of operating parameters, and all the additives needed are already in it. Adding an over-the-counter product to your oil can actually do more harm than good, because the chemicals in the bottled additive may interact with those already in the oil, creating new chemical compounds that reduce the effectiveness of the oil and may even cause damage to the engine.
Conventional Or Synthetic?
It used to be that all motor oils were conventional; that is, they were refined from crude oil pumped from the ground. However, with the development of synthetic or chemically engineered oils in the 1970s, buying oil suddenly became more complicated. Some decades later, it is still the case, since many people don’t really understand the differences and advantages between the two oil types. Adding to the confusion is a new category of oil called a synthetic blend, a mixture of both conventional and synthetic oils.
Crude oil comes from a variety of locations around the planet and varies greatly in composition, depending upon where it originates. Because of this variation, crude is broken down into four general classifications for refining: paraffinic base, naphthenic base, asphaltic base, and mixed base. Automotive lubricating oils are derived from either paraffinic or naphthenic base oils, with most high-quality motor oils being formulated from paraffinic base.
A common misconception of the term “paraffinic” is that it means wax, probably because the wax sold in blocks at hardware and grocery stores is called paraffin. However, the word “paraffin” does not actually mean wax, even though we refer to it by that name. While wax does contain paraffin molecules, the word “paraffin” is derived from the Latin parum affinis, which translates as “not enough affinity.” The term is used to describe the group of hydrocarbons that exhibit limited reactivity, which is why automotive chemists use paraffinic base oils to formulate motor oil. They are less volatile, have greater oxidation stability, and exhibit a higher viscosity index. In short, paraffinic lube stocks are more uniform under the wide range of operating temperatures seen in a modern engine, making them perfect for automotive use.
Many people think the term “synthetic” means that the oil is completely man-made and contains no petroleum chemicals. This is not exactly true. Although synthetic oil is man-made, almost all automotive synthetic building blocks are distilled down from crude oil or natural gas base oil. The most commonly used synthetic base oil is polyalphaolefin, or PAO. The advantage of this type of oil is that it is wax-free, has inherent thermal stability, and offers a high oxidation resistance.
According to Vic Kersey, director for Valvoline’s Ashland Laboratory, the main thing to understand about synthetic oil is that it can be specifically formulated to exacting performance standards. “Synthetic oil means that the base oil is synthetically manufactured as opposed to a crude oil that has been refined down,” he said. “With a synthetic you are designing the molecules to suit specific performance needs. It is, in fact, a designer oil with a number of advantages over conventional oil. It offers easier cold starting, increased horsepower, better fuel efficiency, quicker oil flow upon startup, faster heat dissipation, and improved high-temperature properties.”
These advantages make synthetic oil particularly well-suited for use in extreme environments. Because of the absence of wax crystals, certain synthetic oils can remain fluid to -50 degrees Fahrenheit, and because of their higher resistance to heat breakdown, they also are great for use in performance engines such as those in race cars. Also, because of its inherent stability, using synthetic oil can usually mean extended change intervals. When it was first introduced by Mobil, many experts said that change intervals could be extended from every 3,000 miles to every 25,000 miles. Since then, the “experts” have backed off a bit on those claims, and today few will recommend oil changes less frequent than every 12,000 miles. The thing to understand here is that while it is true that synthetic base stock holds up better for longer periods, the additives in the oil still break down and are used up over time, so frequent oil changes remain necessary. Many engine manufacturers do not recommend extending the change interval beyond that specified for conventional oil, so be sure to follow the recommendations of the vehicle manufacturer.
When synthetics were introduced, they could not be used in all engines, because gaskets and seals in the older vehicles were not designed to handle man-made oils. Vehicles built before 1980 experienced a hardening of seals and gaskets that resulted in severe oil leaks. Today, however, gaskets and seals are much improved, and all oils come with swell agents to make sure seals stay tight and leak-free. So, unless the manufacturer says otherwise, you can assume that a synthetic oil is safe to use in your engine.
Matthew Ansari, a global technology manager for Chevron, says synthetics are a better option for gasoline-powered RV use. “You get more lasting power from a synthetic to start with,” he said, “as they are designed to deliver better detergency and are less susceptible to volatility. And since RVs move around in all conditions, a synthetic’s ability to give the same performance over a wider range of temperatures provides more peace of mind.”
Synthetic blends are a mixture of conventional and synthetic oil and are an attempt to offer motorists a middle-of-the-road choice. Synthetic oils have certain advantages over conventional oils, but they are expensive. By combining the two oil types, it is possible to formulate oil that offers quicker flow upon startup, better high-heat dissipation, and improved anti-wear properties, and still be affordable for the consumer. Blends are available in a variety of mixes, so if you choose a blend, make sure you can trust the mixture of the two oils. Stick with reputable brands. There is no sense in buying a blend thinking you are getting a high percentage of synthetic oil and finding out later that you didn’t.
The Society of Automotive Engineers (SAE) classifies motor oils by their viscosity and the temperature range in which they should be used. They determine viscosity range by testing a specific oil at zero degrees and up to 210 degrees Fahrenheit, assigning a number according to the results of this test. This is called the viscosity grade. Basically, the viscosity grade is a measurement of oil’s ability to flow at a given temperature. Oils with a low number are thinner and continue flowing at low temperatures. Higher number oils are thicker and can flow only at higher temperatures. Once the viscosity for the oil is determined, it is given a grade value by the SAE, such as 10W or 40W. So, at the bottom end of temperature range operation for automotive use is SAE 0W oil, with SAE 50W at the top.
These numbers simply represent a particular viscosity value at a particular temperature range, but they are not an actual measurement of weight, even though that is how we all think of it. In fact, the “W” doesn’t stand for weight; it is short for winter use. Grades for other than winter use are listed in a similar manner but without the “W.” Multiviscosity oils are classified with numbers that represent the low and high ends of the viscosity range for which they are rated, such as a 5W-30. What this means is that 5W-30 oil has the same viscosity as a straight 5W oil when it is cold and that of a straight 30W oil when it is hot. In temperate climates, multigrade oils can be used year-round. By their nature, synthetic oils tend to offer a larger range in multigrade performance, i.e. 5W-40, instead of 5W-20 or 15W-40. If you are contemplating a switch from conventional oil to synthetic, it is important to keep this in mind and make sure any change in viscosity grade is at the bottom end. For example, if you are using 10W-40 conventional oil and change to synthetic oil, you can choose oil rated at 5W-40. Just remember that the top number in a multiviscosity synthetic needs to be the same as the top number in the previous conventional oil.
API Service Classification
The American Petroleum Institute (API) lists classifications for both gasoline and diesel engines on the label with an API service symbol and certification mark. Oil designed for use in gasoline engines has a service classification (currently SJ, SL, or SM), or S-rating, and diesel engines have a commercial classification (currently CF, CF-2, CF-4, CG-4, CH-4, CI-4, and CJ-4). The classifications on a container mean that the oil meets certain performance and quality standards set by both vehicle manufacturers and lubricant companies. The criteria by which oils are classified continually changes, and the API updates its categories on a regular basis to stay current with developing engine technology. This means that a number of classifications are no longer used. If your owners manual says to use an S or C classification other than those listed above, it is calling for an oil classification that is now obsolete and superseded by the next available category. Check with the vehicle manufacturer or the API to see which classification currently meets your engine’s needs (www.api.org).
Gas Or Diesel
Another thing that is vital to know about motor oils is that gasoline engines and diesel engines use different oil formulations, which is why the API has separate service classifications for each.
Eric Olsen, staff engineer for the Chevron Global Lubricant team, says that gas and diesel motor oils are very different. “In a gasoline engine you have virtually no harmful particulate matter formed during combustion,” he noted. “But in a diesel engine, where the fuel sprays into the combustion chamber and is ignited by compression, not all the fuel vaporizes completely, and small, uncombusted bits of carbon particulates are left behind. These mainly go out the tailpipe, and we see it as the tell-tale diesel smoke exiting the tailpipe.
“It also gets into the crankcase,” Mr. Olsen continued, “where the oil suspends and disperses the soot. A good filter traps much of this, but it can’t make up for the dispersion of a quality diesel oil, and if it didn’t have this ability, the oil would thicken, and viscosity would go up with the soot. And you’d get a conglomeration of abrasive soot packets in the oil that can damage engine internals.”
So, can you use diesel oil in a gas engine and vice versa? After all, oil is oil, right? Well, according to oil company engineers, oil is not oil. They say that differences exist between S- and C-classification oils and that you should stick with oil that is classified for your specific engine type.
Matthew Ansari: “Volatiles from diesel oil can damage the catalytic converter and adversely affect exhaust emissions of a gasoline engine. If gasoline engine oil is used in a diesel engine, or vice versa, only in an emergency, then it should be discarded along with the filter and replaced with the correct oil at the earliest opportunity before cumulative damage is no longer repairable.”
Eric Olsen: “Diesel oil has a lot of volatiles in it that will damage the catalytic converter and adversely affect exhaust emissions of a gasoline engine. However, some gasoline engine oils are okay to use in a diesel engine in an emergency, but gasoline engine oil doesn’t suspend the heavier concentration of particulate matter as well in a diesel. Except for emergency use, it is best to stick with oils carrying the correct API service category for your engine.”
Thom Smith, technical director of passenger car engine technology for Valvoline, agrees. “Using diesel oil in a gasoline engine is not the best idea,” he said. “There is 0.11 percent phosphorous in diesel versus the 0.08 percent phosphorous in gasoline engine oil. The extra phosphorous will coat the catalyst, shortening the life of the converter while it increases emissions.
“It is also important to note that gasoline and diesel engines put different demands on oil,” he continued. “Modern gasoline engines are generally smaller and designed to use lighter viscosity oils such as S-classification 5W-20 and 5W-30. A heavier, C-classification 15W-40 is the most common grade for diesels.”
Normal Or Severe Operation
A quick read of just about any owners manual will tell you that the factory-recommended frequency of regularly scheduled oil changes depends upon whether the usage is normal or severe. Determining what normal or severe operation is can be a bit of a problem for many drivers, and the manual isn’t much help beyond telling you that prolonged operation on dusty roads or in extreme climates is considered severe duty.
RVs, perhaps more than any other vehicles, see a wide range of service operations. The average motorhome may sit for weeks or even months on end and then be pressed into quick duty for a 300-mile weekend trip or a 3,000-mile summer vacation that may mean travel from one climatic extreme to another, often while towing a vehicle. It is also not unusual for a motorhome to travel from temperate coastal zones to hot deserts and cool alpine meadows in a single trip. And as we all know, driving conditions can range from bumper-to-bumper rush-hour crawls to wide-open highways that offer few traffic problems but plenty of scenic distractions. Then there is the weather. RVs are operated on mild and sunny spring days, during oven-hot summers, rain-drenched falls, and blizzard-like winters. So what is normal operation?
According to automotive experts, “normal” operation is described as the operation of the vehicle for at least 20 minutes at a medium speed, with a steady throttle and in a clean driving environment. Anything else is considered severe duty. That means virtually all RV operation is considered severe duty, so more frequent oil changes are necessary to maintain the health of the engine. Check the owners manual to find out what the manufacturer recommends as a change frequency for your vehicle, or talk with your RV service professional.
Valvoline’s Kersey points out that the definition of severe and normal may also depend upon the type of engine your vehicle has. “Gasoline engine manufacturers each make their own recommendations for scheduled oil changes,” he said, “with many recommending oil changes every 7,500 to 10,000 miles at a maximum, more often if service is severe. With diesels it is a bit different, and there is often a greater recommended interval between changes.”
Cummins, for example, recommends oil change intervals every 15,000 miles for RVs and more often if duty is severe. However, it is important to realize that this recommendation is based upon ideal operating circumstances, something few RVs ever experience.
Chevron’s Ansari agrees that in terms of drain intervals, diesel engines generally have longer intervals, mainly because of the oil’s formulation. “In terms of drain interval, diesel oil can go substantially longer than gasoline oil,” he noted, “but it is vital to always follow the manufacturer’s recommendations “” no matter what type of engine you have. In general, it is better to change the oil at the end of a long trip and not let it sit in a parked RV for three months, even if you haven’t yet reached the recommended drain interval.”
The Best Oil
Anyone who has gone into an auto parts or discount store has seen the incredible array of oils available to the consumer. There are literally countless brands, dozens of oil viscosities, and various service categories. How can one choose the best oil for his or her rig? Simple; choose the type and service category recommended by the manufacturer and then pick a quality, name-brand product such as Valvoline, Castrol, Chevron, Havoline, Mobil, Pennzoil, or other known manufacturer. Stay away from discount and store brands if at all possible.
Name-brand manufacturers have spent decades building up their company names. They have built quality controls into the manufacturing process to make sure their product is top tier. Their brand is worth a lot, and they don’t want to manufacture a substandard product.
Discount and store brands, on the other hand, may not be up to the same quality as name-brand oils. While it is true that big chains often contract with name-brand manufacturers to supply their product, you can’t always be guaranteed that this is so. Often, discount brands will package oil that has been bought from overstocks that may be old or even manufactured overseas. These oils may not be up to current API quality standards, even when the label says they are.
Valvoline’s Thom Smith says it all: “When you spend a half-million dollars on a vehicle, you don’t want to buy the cheapest stuff out there to protect your investment. It just doesn’t make sense to save a dollar on a quart of oil and take a chance on ruining your engine.”