The following is not based on any scientific evidence, only the experience of a lazy pilot.

In the past 11 years of flying I have never used after run oil and I cannot tell that it has caused any problems. The first couple of years at the end of a days flying I would pump the remaining fuel out of my tank and run the fuel out of the engine. Now I only pump the fuel from the tank, leaving fuel in the engine. None of my engines show any signs of rusting.

None of the small group of guys that I fly with use after run oil and like me they have not experienced problems. The best/safest thing to do may be to use after run oil, but as long as I have good results without it, I will continue my lazy routine.

Eric