There's nothing wrong with adding a few oz. of Castor to Cruel Power
The "All Synthetic" guys get bent out of shape when one suggests adding Castor to an all synth. fuel...why is that? (it's not blasphemy ya know )

Virtually all of the engines that I've seen that were HORRIBLY rusted were run on an all synth. oil fuel. I've owned many hundreds of model engines, and have worked on probably at least a hundred of other peoples engines over the last 5 decades.
Where you fly makes a HUGE difference in whether you have rust...
If you live / fly in an arid climate, the chances of you encountering rust in your engines is considerably lessened. Where I live, it can be very humid for long periods of time...that really makes a difference.

All that being said...run whatever ya want...it's only a hobby...your life doesn't depend on it...have fun.[8D]