While castor will EVENTUALLY do some gumming of the engine, the additional protection against a lean run may be worth it to you.

I usually add 3 oz of castor to 1 gallon of fuel (Powermaster 15% ). I have 3 Saitos, a 72, 80GK and an 82. My 72 has about 4 gallons through it and the only thing I have had to do is one valve adjustment and 2 glow plugs.