I've taken the liberty of copying snippets from numerous articles that help to understand the differences between the various oils and what you may want to consider when selecting an oil for your engine, whether it is a VW, Corvair, Continental, or Lycoming.
Synthetic Oils: Shell Oil tested all-synthetic oils in aircraft engines, and what they found wasn't good. At 600 to 900 hours, the engines began to burn more oil and lost compression. "When the engines were disassembled, we found that the piston rings were covered with a gray tacky substance that was primarily made up of the lead by-products of combustion." This is the reason why the full Synthetic Aviation oils disappeared off the market 20+ years ago. Once the lead is gone from Aviation fuel, synthetic oils may become viable for Aviation engines again. This is also why AeroShell uses a Semi-synthetic oil, which is only 30% synthetic. That's as much synthetic as they could use and not have lead sludge left in the rings. Automotive Oils: Air cooled aircraft engines were designed before additives were available and have not really changed much over the years. When ashless dispersant oils were introduced for auto engines, they were also suitable for aircraft engines and eventually were adopted for aviation use. However, when zinc antiwear and metallic detergents were formulated into auto oils, an important divergence occurred. Aircraft engines burn a fair amount of oil and, if these metal-containing detergents and antiwear compounds are present, they can form metallic ash deposits in the combustion chambers. These deposits can lead to destructive pre-ignition, which could burn holes in the tops of pistons with obvious catastrophic results. For that reason, it was decided that aviation oils were to remain ashless to avoid the risk of metallic deposits. The benefit of using ashless dispersant oils is, obviously, a cleaner engine. Aircraft engines would also benefit greatly from the addition of other automotive additives such as anti-wear, detergents, and corrosion inhibitors, but the downside is added cost. Ashless versions of these performance additives can cost up to 10 times more than standard ash-containing additives. Aviation Oil: The Ashless dispersant rating really is important to any aviation engine. As outlined above, there are no metal based additives in the oil, so it will not leave ash clinkers behind as it burns. Additionally, 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. Being mineral based, or at least 70% mineral based oil in the case of Aeroshell 15W50, it will keep the lead sludge cleaned out of the rings if you are burning leaded Avgas. ? My conclusion here is that if you want to use an automotive oil in your aircraft engine (VW or Corvair included), you would want an automotive oil with an AD rating on it as the Ashless Dispersant will not leave ash as it burns that can create hot spots that can cause pre-ignition/detonation. However, I don't think you can find a modern automotive oil without metal based detergents, so none have an AD rating. The metalic anti-wear compounds are great in a tight toleranced water cooled engine. Not so much in an air cooled engine. However, they will likely work well as long as the engine isn't burning any significant amount of oil which is, of course, what leaves ash in the combustion chambers. One of the questions someone is sure to ask is, "Doesn't burning leaded fuel leave ash in the combustion chambers that could cause detonation?" The answer, "Absolutely."? Leaded fuel leaves a lot of ash.? But it is a continuous process where lead salts are left behind as a combustion byproduct, but also sloughs away and goes out the exhaust.? The lead deposits in the combustion chambers is one of the reasons why the timing is so conservative on aircraft engines and the octane requirements are so high. The combustion chambers get significant lead deposits that can cause detonation. However, compounded with other metals the deposits are likely to be much harder and may not slough off the way the lead sloughs from the combustion chambers. I overhauled an engine a couple of years ago that had an aftermarket ignition system on it and the owner wanted to press the timing margins a bit without really understanding the timing curve of the ignition product he was using. The engine went into undetected detonation. The only indication was that his CHTs started climbing above normal. As the CHTs continued climbing the engine went into a runaway pre-ignition with his CHTs over 550?and oil temp above 250? before he could get it on the ground. The only way to get the runaway pre-ignition to stop was to cut off the fuel to the engine, which was not a viable alternative until he could get to a runway. The engine suffered significant damage with severe erosion of the ring grooves. There was no need to clean the heads or valves as any lead residues had been blasted away by the detonation. The rod bearings also suffered some scuffing damage as the detonation beat the rods into the crank enough to penetrate the oil film in the bearings. My conclusions: I know many of you are running synthetic and automotive oils in your Automotive based engines. Nothing wrong with that, especially if you are running mostly Mogas and your engine doesn't burn much oil or push the margins for detonation. Just know that if you use synthetic oil and burn leaded Avgas, you may be significantly shortening the life of your pistons, rings, and cylinders. If you are using an automotive detergent oil, you may be leaving some metal based ash compounds in the combustion chambers that could cause detonation if you are pressing the detonation margins. Depending on how much leaded fuel you burn,you may also be transporting some lead salts through the engine with the oil that will continuously polish the bearings and cylinders, which will contribute to excessive wear, although I suspect with regular oil changes, any additional wear would likely be minimal. Once the lead is gone from aviation fuels, the range of oils available for use in Aviation engines should really open up and the synthetics should be viable for use again. Based on experience with automobiles as well as my own two aircraft that rarely see leaded fuels anymore, I expect aviation engines to last significantly longer once we get the lead out. The final certification of unleaded aviation fuel is supposed to occur in 2018 with 100LL going away some time after as production of the replacement fuel(s) ramps up. -Jeff Scott Los Alamos, NM ?
