My experience is the following.

I have a Thunder Tiger F-75S....the first 5 liters of fuel were 22% Castor oil, 12% Nitro and the rest Methanol.... and ran very good.

I've tried coolpower 5% and the engine didn't like the fuel at all!

I tried another fuel with 15% syntethic oil and 12% nitro and the engine didn't produce as much power as with the castor oil...

The problems you may experience running castor oil on 4 strokes are normally not related to performance, but related to carbon build up on the exhaust valve....you may have to clean it regularly (de-coke).

You don't have to use 20% neither... With about 15% Castor oil you will be fine.

The engine may just run fine without nitro as well...maybe you won't get a very low idle or super clean transition...But I haven't tried my 4 stroke without nitro.... I've run all my 2 strokes always without nitro.

Just give it a try! it won't hurt anything in the engine you plan to buy!