After-run Oil as in the ones meant specifically to place in your engine will absorb moisture. This is one reason to stick with them vs other oils that you may deem worthy of this task. Some time was spent to ensure that these oils work best with your motor , dont foul up the glow-plug and allow it to prevent rust.
Second , when you take your glow-plug you must clean the heatsink area of any debris otherwise your going to introduce sand into the threads of the heatsink or engine itself. Causing leaks and or engine damage anyhow.
So before doing so take this simple caution either with compressed air , nitro cleaner or just plain ol taking it off and spraying down with wd-40 and wiping it with a q-tip or removing the heatsink alltogether then just wiping around the button head and re-installing then remove glow-plug.
This sounds tedious and I myself have pretty much quit using after-run oil. Ive just switched to the run the motor out of fuel. Then put it back on the starter box , place glow-starter back on and try to start it again. This will normally fire it back up and itll run the last drops of fuel out. Keep the glow-starter on it. I also put my finger on the exhaust tip a few times to help push anything in the lines through.
Motors seem fine to me and its just plain easier to do. But for a little extra security and when the motor is clean I do add in ARO normally asap after a race. But I dont do anything like this during a race or after a qualifier or anything. Just run it down and let it sit till the next race.