I'm pretty sure that 2 stroke fuel, has a higher percentage castor oil in it. While, on the other hand 4-stroke oil has more synthetic. It's my understanding that the synthetic oil breaks down cleaner/better. The castor oil leaves more of a gummy residue that could eventually mess up your valves.

Personally, I have 5 4-stroke engines and use nothing but 4-stroke fuel with 15% nitro. I've never had a problem. I have used 2-stroke in a pinch. On a Sunday say, when my LHS is closed and I've run out of 4-stroke fuel.