If you own an airplane, using the right oil and knowing when change it are some of the most significant things you can do to keep an engine running to (or beyond) the manufacturer's recommended TBO.
Figuring out which oil to use requires understanding a few concepts—notably, the type of oil (straight mineral, ashless-dispersant or synthetic), viscosity and additives. Which type to use also depends on what kind of engine you have and what kind of flying you do. I'm going to start with the traditional horizontally opposed piston engines used in most GA airplanes, and then move on to more exotic alternatives. But first, a warning: Before changing the oil in your airplane, check the owner's manual! It may specify particular oils. More likely, it will provide general guidance, or refer to a service bulletin (SB) or service instruction (SI) from the engine manufacturer.
Most piston aircraft and engine manufacturers approve two major types of oil for use: straight mineral oil and ashless-dispersant (AD) oil. Straight mineral oil is pretty much what comes from a refinery, and it's a good lubricant, but it won't keep your engine clean. During break-in, that can actually be a good thing! Ben Visser, a former AeroShell lubrication expert, told me that using straight mineral oil for break-in "goes back to the old hard chrome used to bring cylinders into specification, where you had to actually wear down the parts—the particles worn off work as a lapping compound."
On the other hand, constantly polishing the inside of the engine isn't ideal for long life—as you polish, clearances increase, which eventually will lead to oil loss, and debris from polishing has to accumulate somewhere. That's where AD oil additives come in: Instead of allowing particles of metal to agglomerate (or burn down to ash), AD oils disperse metals (and other contaminants) and flush them out of the engine at the next oil change. So, AD oil is almost always recommended for use after break-in. In some cases, AD oil is recommended during break-in!
Both straight mineral and AD oils are rated for viscosity—a measure of resistance to flow, or in practical terms, the "thickness" of the oil, which depends on temperature. Viscosity is expressed by the "weight" of the oil, measured on either of two scales—the familiar Society of Automotive Engineers (SAE) or the commercial aviation scale (the latter is basically double the former). Typically, you'll use a higher-viscosity oil at higher ambient temperatures. For the Continental O-470 in my Skylane, Cessna recommends SAE 50 (or 100W) above 40° F, SAE 30 (or 60W) at lower temperatures.
There also are variable-viscosity oils, which are thin at low temperatures but thicken at high temperatures. A 10W-40 oil is an oil that acts like SAE 10 at room temperature, but like SAE 40 at operating temperature. That's ideal for pilots who live in the snow belt; you don't want to try starting the engine at below-freezing temperatures with an SAE 40 oil. On the other hand, the SAE 10 oil that's good for starting won't do a good job of protecting your engine on a long cross-country.
Variable viscosity oils start with a low-viscosity base oil (for a 10W-40, starting with an SAE 10 base oil) with viscosity modifiers (VMs) added. Think of VMs like small balls of yarn. At low temperatures, they have no real effect on the overall viscosity of the oil, but as the temperature rises, the yarn (actually polymers, basically, plastics) unroll, and as they do so, the oil thickens. Most variable-viscosity oils are built on natural mineral base oils, but there also are fully synthetic lubricating oils that offer several advantages over mineral oils (among other things, longer oil change intervals). They're not used in piston-aircraft engines because the lead from aviation gas can interact with the oil to produce sludge, which needs to be removed by changing the oil.
Other types of engines, particularly turbines, don't use leaded aviation gas and can use synthetic oils. Which brings up a big difference between synthetics and conventional mineral-based oils: Conventional oils can be freely mixed. If you're flying a piston-engine airplane on a long cross-country and find yourself a quart low, you don't have to top up with the exact same oil—any aviation oil with a viscosity approved for your engine will be fine. Not so for turbine engines. Different manufacturers may use completely different synthetic chemicals, and mixing them can have unpredictable results. Even oils from the same vendor may be incompatible. Another significant difference is oil-change interval. For piston-engine airplanes using mineral-based AD oil, the interval is usually listed as 50 hours (if you have a replaceable oil filter) or half that (for older airplanes that only have an oil screen). But the oil manufacturers add an additional limit: You should change the oil at least every four months, regardless of how many hours have been flown. Why? Because oil picks up water and acids. According to ExxonMobil, "Changing your oil on a calendar schedule as well as a flight hour schedule helps ensure you get contaminants out."
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To get rid of water, you need to run the engine, and not just at idle power. An oil temperature of at least 180° F is required to drive water out of the engine. Getting the engine that hot will require flying it, at least for a couple of touch-and-goes. Visser also warns that many analog oil temperature gauges are poorly calibrated—just getting the needle into the green may not be enough. He recommends having a mechanic calibrate the temperature bulb and add a paint mark to show exactly where 180 degrees is on the gauge (those of us with digital oil temperature indicators, on the other hand, don't need to do this). If you can't get the oil temperature that high, check with your mechanic to see if a winterization kit is available, which will block some airflow to the engine.
If you won't be flying an airplane at all for an extended period, consider switching to a preservative oil, such as Phillips 66 Aviation Anti-Rust 20W-50 or AeroShell Fluid 2F. They're both fly-away oils that don't have to be drained out before the airplane is flown, but you'll want to replace them with regular AD oil after no more than 10 hours of operation. For airplanes flown infrequently (once or twice per month) in high-humidity areas, Phillips recommends mixing up to 10% Anti-Rust 20W-50 with a conventional AD oil (such as Phillips 66 X/C 20W-50) to get a combination of corrosion protection and AD oil performance. On the other hand, ExxonMobil, which doesn't offer a dedicated preservative oil for aviation use, builds a rust inhibitor into their ExxonElite 20W-50: "Moisture may still diffuse through both the oil layer and the rust inhibitor layer, but it will take longer because of the water-repelling nature of the additive, which offers you greater protection against rust." Jerry Toenjes, an AeroShell sales team leader, pointed out that his company's W80 Plus, W100 Plus and 15W50 oils "include Lycoming L-16702 anti-corrosion additive in the proper concentration" that "helps to protect your engine in sporadic, limited use."
Some engines need additional additives. Lycoming's O-320-H requires a phosphorus-based "anti-cuffing" agent that reduces cam wear. It's available alone (Lycoming LW-16702) and is also blended into oils from several major vendors. According to Visser, it may reduce wear on infrequently flown engines.
Now that we've covered the ins and outs of conventional piston-engine oils, what if you're flying an ultralight, light-sport, or diesel engine airplane? The same basic rule applies: Check the owner's manual. It will recommend an appropriate oil for your engine. AeroShell offers oils for two-cycle and four-cycle engines used in ultralight and light-sport aircraft and a fully synthetic diesel oil for use in engines such as the SMA diesel Cessna now offers as an option in new Skylanes. Toenjes told me those oils use different technology and shouldn't be mixed with conventional mineral-based aviation oils. He added that AeroShell is using a fully synthetic technology in its diesel oil because lead isn't an issue for those engines.
One more thing to consider is oil analysis, in which a sample is collected during an oil change and shipped to a lab for evaluation. Phillips 66 Lubrication Engineer Steven Strollo told us: "Oil analysis accompanied by engine oil filter examination for any engine regardless of drain interval is suggested to detect and correct operating problems, recognize poor maintenance or repair practices, identify excessive operating conditions and increase engine life." ExxonMobil goes further: "Send a sample of your oil to a qualified laboratory every 60 days whether you've flown much or not, and ask them to test it for water content. If the lab says you have water in your oil, you may need to raise your sump temperature or change your oil more frequently."