When you put away your engine for the day (or whatever time), there is always going to be some amount of fuel left in the engine (carb, crankcase, and cylinder). The nitro (it might be the methanol, but either way this statement applies) content which is in EVERY fuel contains is hydrophoric. This means that one of nitro's chemical properties is that it naturally attracts water. Water which is found in the surrounding air (humidity) starts to pool in the engine, and as you can guess water + metal engine = corrosion. Below is a quote from a Traxxas Manual which describes a proper after-run procedure (this applies for any nitro engine of any design or variation)
1. Whenever possible, shut off the engine by pinching the fuel line
closed. This allows most of the excess fuel to be consumed by the
engine. Be sure the throttle is in the idle position. You may have to
pinch the fuel line closed for several seconds before the engine stops.
2. Completely empty the fuel tank. Use your fuel-dispensing bottle to
suck out the old fuel. Do not mix the old fuel with your fresh fuel
supply. If you leave fuel in the tank, transporting or handling your
Revo may cause fuel to run into the engine.
3. With the fuel tank empty and the throttle at the idle position, try to
start the engine. The engine will most likely start and run for a few
seconds as it uses up any fuel remaining in the engine and fuel lines.
4. Once the engine stops, clean the outside of the engine with
compressed air or spray motor cleaner. Once the engine is clean and
dry, remove the glow plug power wire, glow plug, and air filter.
5. Open the throttle fully and spray a one-second burst of WD-40 into
the carburetor and into the glow plug hole (Caution! Wear safety
glasses to prevent spray from getting into your eyes). If you are using
after-run oil, follow the manufacturer’s instructions.
6. Place a rag or paper towel over the engine to catch any WD-40 or
after-run oil that may come out the carburetor or glow plug hole.
7. Connect the EZ-Start controller to the model and spin the engine for
10 seconds.
8. Remove the rag or paper towel and repeat steps 5–7 two more times.
9. Clean and re-oil the air filter so it will be ready for use next time. See
page 23 for air filter maintenance instructions.
10. Replace the glow plug, reconnect the glow plug power wire, and
reinstall the air filter.
The idea is to first burn as much fuel from the engine as possible. Then the WD-40, as you all should know, displaces the remaining fuel. Using this method 99% of the fuel from your running session will be expeled from the engine. Traxxas then adds that you may also use after-run oil to further protect the engine.
Now I will address the incorrectness of Morgan Fuel's statement:
Petroleum based !QUOT!after-run!QUOT! oils do not mix with methanol-based fuels. So, we definitely don’t recommend any petroleum based lubes.
These petroleum based lubes are meant to DISPLACE the remaining fuel, therefore they are NOT supposed to mix. They are true in saying they dont mix, but wrong in their argument. No matter what they do to their fuels, there will always be methanol and nitro which attract water. You cant stop this attraction, so the only option is to expel the fuel.