This isn't correct because it's the methanol that's hygroscopic. Nitro can only absorb something like 2% of its volume of water. Methanol though will just keep on absorbing water right up to 100% of it's volume. If you shut the engine down at the end of the day without first burning off all the fuel that collects inside the crankcase then the remaining methanol will start absorbing water out of the air. The methanol then evaporates much faster than the water leaving raw water inside the engine.

Synthetic oils don't "stick" to metals very well, they slide off over time leaving the bare metal surface to be in contact with water which then causes rust. Castor is what's called "polarised" which means that the castor molecules are attracted to metal kind of like a magnet to iron. It can't slide off so it leaves a permanent barrier against any water. A proper after run oil is also polarised which is why they work and are really only necessary if there's no castor in the fuel.

Nitro is just an additive to the base fuel and any engine will work quite well with zero nitro. To the best of my knowledge there's only one engine made that's had it's compression altered specifically for the American market's fascination with nitro and that's the Mokis sold as Mark engines. Some engines (early Saitos for instance) were made to run at their best with zero nitro but began to get a bad reputation in America because no one could get them to run properly when they were fed the usual high nitro diet (operator error) so they had to be altered.

The only possible reason for using high nitro (or even any nitro) is to try to squeeze every last few revs out of an engine. If you need nitro to make the model fly properly then you simply don't have a big enough engine in it.