Its accepted wisdom that your engine will last longer if you change the oil frequently. If thats true-and were as guilty as anyone for supporting whatever mythology applies-isn’t changing it more frequently even better? And what the heck does “frequently” really mean? Engine manufacturers have their own recommendations, which are sometimes hard to find and even harder to follow. But as far as we can tell, these are determined less by actual research-based findings than they are textbook recommendations from engineering manuals. So for this report, we set out to find out what the piston-engine aviation oil professionals who
manufacture and sell the oil and the people who analyze the dirty stuff have to say. What do they recommend for the typical aircraft owner?
Why Change Oil?
Does the oil actually wear out? No, it doesnt, at least not appreciably during a typical interval between changes. Instead, we change oil to remove potentially damaging contaminants. After sitting for hours, days or weeks, your engines oil is heated up, pressurized and circulated past pistons, crankshafts, camshafts and cylinders. As it circulates, oil is collecting tiny bits of metal as, inevitably, your fresh overhaul wears and turns itself into a high-time engine. Those tiny metal particles suspended in the oil are normal and, when analysis is performed, their characteristics help us determine if any components are wearing abnormally. Some are caught by the filter, fewer by a screen and those small enough to escape both wind up at the bottom of the crankcase or are too small to cause any damage, according to Conoco-Phillips Harold Tucker, who is director of product technical information and training. As long as you don’t fly in a sandstorm, or you arent an ag operator flying at low level, internal abrasion from those particles isn’t a problem to get overly concerned about, Tucker says.
But thats not the end of it. While the oil is collecting various bits of expensive metal, its also picking up the byproducts of combustion. Dating to the 1930s, technology in air-cooled powerplants has been designed for looser clearances when compared to modern automotive engines. “A tremendous amount of stuff gets by pistons into the crankcase,” says Ed Kollin, technical director of Aircraft Specialties Lubricants, which makes CamGuard oil additive.
After a few hours of operation, the oil in the crankcase entrains a wide range of contaminants. For example, in addition to partially burned hydrocarbons, there’s raw fuel from overpriming and rich mixtures, plus the biggest problem of all: water, itself a combustion byproduct. That water, combined with the chemical stew from thousands of tiny explosions in the cylinders, forms acids. Those acids promote corrosion and, at least anecdotally, we believe this is the single most significant wear item for aircraft engines. “Corrosion, as a result of infrequent use, is by far the greatest problem in those engines Ive examined that have problems,” Kollin says.
Between Changes
Instead of worrying about the tiny bits of metal in the oil, which show up at analysis-your engine is on analysis, right?-we really should be worried about corrosion. While modern aviation oils do a good job of lubricating and are good at minimizing corrosion, combustion byproducts eventually overcome even the best oil chemistry. And corrosive acids, according to Kollin, can start showing up in as little as 20 hours of operation: “After 20 hours, the oils chemical equilibrium shifts to a corrosive environment,” he says.
Short of changing the oil, the presence of contaminating chemicals and water is the main reason piston aircraft engine gurus urge pilots to make sure their oil is