It seems like every week someone asks if after-run oil is really necessary, and the answers range from "yes, after every run" to "only if you're going to let it sit for weeks" to "never, it's a waste of money". The premise is usually related to preventing corrosion, but that's not really my concern.

I have a tiny little .08 engine that I drive like I want it to explode, and I'm fine with it because the vehicle is fun that way. However, priming the fuel feed after it's been sitting for more than a couple days is a huge pain, because any residual fuel in the crankcase evaporates and the tiny amount of oil left behind isn't enough to form a good seal between the piston and sleeve. (the engine still runs great, but there's basically no pinch left, and the tiny displacement makes it difficult to generate enough suction to prime the fuel feed using just a pullstarter.)

I'm sure I'm the millionth RCer to have figured this out, but it can't hurt to post it again for any noobs out there: Putting a couple drops of after-run oil into the combustion chamber before trying to prime the fuel feed makes a huge difference, because it restores the fluid seal around the piston. Some might point out I could do the same thing with fuel, but fuel doesn't come in convenient dropper-bottles, and I don't feel like buying a bottle of oil just to empty it out and refill it with fuel. Plus, since after-run oil also makes a good rebuild lube, it just doubles the reason to keep some around, even if you don't use it during long-term storage.

True. Oil is not only for lubrication but also a better seal, I call it a hydrolic piston ring. That is why some worn engines do run fantastic on a fuel with a bit more oil. A good example is with the airplaine guys who do run an engine for decades without issues, the standard 18~20% oil does the trick.

Remind me, why does aircraft nitro fuel have so much more oil in it? Is it because the engine runs at high RPM for long periods of time, or is there another reason?

Basicly thats it. Also due the low internal timings the engine is less sensitive to the fuel. Running airplane fuel in a car engine can cause tuning issues because the amount of oil slowing down the speed of the flow.

Ah, I didn't realize airplane engines were also timed differently. I figured they were the same designs with smaller heatsinks.