In a ringed engine, a straight-castor lube package *can* cause the ring to stick after a while but on an ABC engine I know of no instance where such a fuel has caused any problems (unless the engine is left unused for any length of time).

However, with good synthetics on the market there seems to be little reason to run a pure-castor lube package these days and most engines will show a useful power-gain when a good proportion of synth is used in the lube package (due to the lower viscosity and viscous drag that results)

Of course plain-bearing (bushed) engines should still have a very healthy percentage of castor (because you want hi-viscosity to extend the journal-life).

On my BB engines I run just 2%-3% castor with the balance synthetic, on my plain-bearing engines I run at least 10% castor with the balance synthetic.

Castor is a fine lubricant but if you want the *best* performance from your engine then a little synth will not hurt one bit at all.