I think the expansion rates of a given metal is highly dependent on the specific alloy.
Ive been using all castor in most of my engines for many years now. Oddly enough, the only engines I find varnish in are my ABC arc car engines. None of my aircraft engines regardless of cylinder/piston material, have any varnish. Carbon, yes... And FWIW, I choose the oil content moreso based on displacement than cylinder construction. Smaller engines get more oil than larger engines with one exception being sleeve bearing cranks. Those engines seem to run better with more oil; the caveat is with the higher compressed engines, higher oil content usually requires dropping the nitro a bit. For a .40 - I'd use 20% oil. If using a predominately synthetic fuel, I would add some castor and drop the nitro a little bit. In a Fox - if it has any head button besides the cone shaped low compression heads, I wouldn't use more than 5% nitro anyway. I have a low nitro head and a high nitro head for a compact .40, but I never tried using more than 5% with the high nitro head.