I bought my first airplane, a Globe Swift, from a retired petroleum engineer and A&P mechanic who had lavished hundreds of hours on his airplane. In fact, he refused to sell it to me unless I agreed to maintain it his way.
Ed Kaston lectured me several times that the typical horizontally opposed piston aircraft engine is a complex mechanism with parts thrashing in all different directions, trying its best to tear itself apart. He explained that the only thing preventing that destruction was oil, and that the best preventive maintenance measure I could possibly take was simply to change the oil every 25 hours or three months, whichever came first.
He also suggested I not allow the airplane to sit for long periods of time without running it for at least 30 minutes around the pattern. That would bring the oil up to an adequate operating temperature to burn off the contaminants generated by lack of use.
I stuck to petroleum expert Kaston's advice religiously, even hired him to do the annuals, and true to his promise, the little 125 hp Continental ran perfectly for the eight years and 800 hours I owned the airplane.
Oil is no longer as cheap as it was in those days, but it's probably still the least expensive and most effective preventive maintenance measure I can take. Accordingly, I've become something of a student of aviation oil, and as a result, I change it every 20 to 25 hours, guaranteed.
In the perfect engine, oil would keep every part from ever contacting any other part. Without metal-to-metal contact, there would be essentially no wear, and a piston engine might last 5,000 to 7,000 hours or longer between overhauls rather than the typical 1,000 to 1,500 hours we see today.
In the real world, there's always some metal contact, because oil has a limited film strength. If two metal surfaces are thrown together with high enough temperature and pressure, even the best oil will break down, and the metals will begin to meet and wear.
The more frequently an airplane runs, the longer it's liable to continue running. The late Lyle Shelton, primary builder, owner and pilot of the unlimited F8F Bearcat Rare Bear, rebuilt his warbird in the hangar next to mine in Compton, Calif., and Lyle kept a Cherokee 140 that his crew and I used for shagging parts or any other business on the Bear.
Lyle's Cherokee sometimes flew an hour or two a day, and he used to change the oil an average of about once every two weeks. The O-320 Lycoming ran perfectly for nearly 3,000 hours, or 1,000 hours past its TBO, flying 20- to 30-minute hops around the Los Angeles basin. Even so, when the crew finally overhauled the little engine, the cylinder walls, bearings and all the other greasy bits looked practically new.
That's partially because Lyle used the best grade of aviation oil he could find. As most aircraft owners are aware, automotive oil isn't appropriate for aircraft piston engines. Auto oil is designed for engines that are water-cooled and built to very tight tolerances. Air-cooled engines are constructed to looser tolerances and are specifically designed to burn oil. That means auto engines operate at much lower temperatures than do aircraft powerplants. Oils specifically intended for auto engines will break down in the high temperatures of aircraft engines. These oils leave metallic ash additives, which, when burned in a combustion chamber, leave deposits that could cause detonation.
If you need to add a quart or two of auto oil in an emergency, that's normally not a problem, especially in high-capacity sumps of 11 or 12 quarts, but a regular diet of motor oil as opposed to aviation oil is setting you up for a fall.
There are two types of aviation oil, mineral or ashless dispersant, the latter sometimes incorrectly referred to as detergent oil. Mineral oils are preferable for breaking in new or overhauled engines specifically because they have no additives and allow the engine to "set" or stabilize oil burn before switching to ashless dispersant oil. The rule is you can add a quart of mineral oil to a service of ashless dispersant, but don't try to go the other way.
Though it may sound like a cliché, oil is quite literally the lifeblood of any piston aircraft engine. Anything that interrupts the flow of oil is almost guaranteed to cause problems. For that reason, it's important that the paper filter be replaced or the metal filter cleaned every time you change oil. A proper oil service may therefore demand slightly more oil than what the book calls a full sump, because you'll need to refill the filter. An eight-quart engine may actually demand 8.5 or even nine quarts to bring the engine to full. Conversely, the engine may tend to blow out the top half-quart of oil. A little too much oil isn't necessarily a bad thing.
Too little can be a major problem. For the pilot, fluctuating oil pressure could be a sign that the engine is running low on oil. This may mean air bubbles are being fed to critical bearings rather than lubricating oil, causing unnecessary wear.
Conversely, you may have little or no warning that you're low on oil until it's too late. I once delivered a new Cessna T-Stationair to Sydney, Australia, and on the initial 14-hour overwater leg from Santa Barbara to Honolulu, the engine burned 10 quarts of oil. All temperatures and pressures remained normal, and the big Lycoming ran great for the whole trip. The next morning, when I tried to check the level, it was off the bottom of the dipstick. Cessna pulled the engine and replaced it in Honolulu, and I kissed the ground for my good luck. Had the leg been 15 hours...?
Oil temperature is another critical concern, and both too hot and too cold can be preludes to trouble. Every pilot knows to watch for high oil temps, but how many pilots have seen temperatures off the bottom of the gauge? A few years ago during an untanked winter delivery of a Cessna 400 (now Corvallis TT) from North Carolina to Geneva, Switzerland, I saw oil temps I couldn't believe. I was flying the leg from Bangor, Maine, to Goose Bay, Canada, in January, and the digital oil temp on the Garmin G1000 was showing me readings consistently between 60 and 70 degrees F. Most aircraft flight manuals prohibit even runups until oil temperature reaches 100 degrees F. At the time, OAT was about -40 degrees F, and I couldn't find any way to keep the oil warm. It turned out there had been a service bulletin issued by Continental requiring part of the cowling air intake to be blocked in extreme cold weather. That bulletin hadn't been complied with on my airplane.
On the surface, aircraft oil seems such a simple product, but my friend Ed Kaston, taught me differently. Every time I change oil on my Mooney, I replay Kaston's advice about the importance of proper lubrication in an aircraft engine. The alternative isn't even worth considering.