Sport Pilot is right about corrosion inhibitors. Allow me to add some additional info.
1) Anti corrosion additives are only hydrocarbon based: FALSE
These are generally metallic in nature and can consist of any or all of the following groups. Metal Deactiviators (MDA usually amine based), Extreme Pressure Additives (Phosphate based), Anti wear additives (ZDDP or even POE based) PAG itself canbe used as anti war because it is highly polar but has low internal shear. Then there are other Esters as well.
2) Water absorption of PAG oils is bad and will corrode the engine: FALSE.
In an aluminium crank case the water absorption is beneficial in that it promotes the formation of the AlO2 (aluminium oxide) layer which prevents further corrosion of the aluminium. The steel components are generally unaffected due to the coating of PAG, and other polar additives in the lubricant.
3) water absorption into alcohol fuels is a bad thing and will cause rust : FALSE
Its been scientifically proven that water absorption actually reduces the corrosivity of alcohol fuels by absorbing negatively charged ions. As always it the degree of water absorption. Submersing the engine is an obvious case where water is a bad thing.
I suspect that there are other reasons for the corrosion many see in their engines.
1) rust is already present in the metal used for the steel components of the engine. Poorly processed steels and veryhigh carbon steels are prone to surface corrosion no matter what we do to them. Lets face it, hobby engines are low volume so the steel used is going to in proportion to the final cost of the engine...
2) Poor quality methanol is being used
3) dirt /contaminant ingestion via the engines carb would be trapped in the crankcase and if there are salts i.e. flying in an area close to the ocean or salt pan or salt lake,
4) Copper ions from poor quality brass fuel tubing, brass clunks etc. I tend to use aluminium and steel components in my fuel systems.