Frosty,

Just don't use any more than 5% nitro, or you will be cursing yourself. If your LHS doesn't stock it, drive further away, or order in some 5%, or FAI.

About the oil; although MVVS specify 20% Castor oil (no synthetic), The Morris Hobbies web site says you can use synthetic oils.
Some prominent RCU members, affiliated with MVVS said the Motul Micro and Graupner Aerosynth II synthetic oils, can be used in percentages of 15.

I still run 20% total oil in my MVVS engines. For safety I suggest some Castor oil should always be (at least 3%).

Sincerely,