A couple years ago, an old flyin' buddy (he's 80 and kinda old fashioned...likes a castor based fuel ) and I tried an experiment and ran his Saito 1.80 on an all castor fuel. On about the third gallon the exhaust valve started sticking...the stem was varnishing up. We first noticed a problem when occasionally, it had no compression (the valve was hanging open a bit from time to time ) gradually the problem got worse, until we got fed up with it, so...

I disassembled the jug and cleaned the valve (de-coked or de-varnished what ever ya wanna call it ) and it ran good as new.
We quit running it on all castor though, and went back to powermaster std. sport fuel...which is 18% oil (2/3 synth and 1/3 castor )

Both fuels were/are 15% nitro.