Hi!
As you're from the right side of the pond why don't use 15% all synthetic oil like the rest of us is using in Europe!?.
Hardly anyone over here in Sweden are using castor oil these days.
Haven't you heard of the Germans developing new synhetic oils 10-15 years ago for modelengine use!?
15% oil is the norm over here and many use even lower percentages.
The German Aerosynht 2 and Aerosave from Fuchs, has been the most popular brands used here.
I myself use the French brand Motul...You have probably have heard of if from the motorcycle racing world or from F1 racing. Motul has a special synthetic oil made for us glow adicts called Motul "Micro".

Why Just engines in Britain advokates 20% castor oil and not synthetics pussles me??
Being the Swedish MVVS distributor I recommend 15% all synthetic oil for all glow engines.

Regards!
Jan K
Airracing products

http://hem.passagen.se/airracing