Charlie,
I'm curious... You started off by describing your engine as fitted with the factory ABC P/L. Seems most of the responses ignored that. I hope the Fox factory instructions take account of the different oil needs for ABC top ends...
The main bearing is not the reason Fox 35s traditionally need ~28%+ castor: the leaded steel sleeve and Meehanite cast iron piston pair is.
Conclusion? You could certainly get by with a 10%nitro/22%castor/3%synthetic fuel, and possibly with the universal modern ABC/AAC/ABN juice: 10%nitro/11%each castor & synthetic.
Castor delays the break-in of a well-made "ABC" type P/L. The working bits need to reach designed temperatures to expand to the optimum running fit. Castor carries away heat, as it (usually) won't burn at our running temperatures.
Castor also tries to deposit scorched oil on the piston, at least. Again, this is counter to the intention of the "ABC" top end. CLEAN surfaces, running at proper temps, is what they are intended for.
Now, I haven't run a factory fitted ABC Fox 35, so it may be worthwhile to see if this idea has been tried, and if so, how well it worked. I have heard good about the Fox 35 ABC - lower vibration, particularly.