Hi!
I have never in my 33 years flying pumped back fuel! Totally unnecessary!!!
Over here in Europe many of us use 15% or less all synthetic oil like Motul "Micro" , Aerosynt 2 and 3 from Fuchs in Germany and many other oil companies and these oils contains rust inhibitors.

As I'm responsible for our clubs 3 training airplanes, were we use 15% all synthetic oil and 5% nitro, I have never during the last 5 years never seen any rust in the engines what so ever. These engines are just fueled and flown and never do I pump back fuel after a flight. The planes are stored in a metal arms locker between flights and stands there,some for months between flights, and never any rust.

Many times in the past when people ran Castor oil and hadn't used their engines for months or years the rust problem was severe in all engines, not to speak of the problem with Castor oil oxidizing, gumming up ball bearings.

As I never use and "special" after run-oil. I do now and then oil in my engines using ATF oil or just the same all synthetic oil I mix in my fuel.
And I pour it in! Don't be bashfull ! Pour in plenty of oil into the carb! A tea spoon is nothing! Synthetic glow fuel oil or ATF oil doesn't attack O-rings or silicon rings in the carb.