You do not need more than 25% oil in your fuel even for the iron/steel technology in the older Fox motors. Most of them will run well on fuel that has some synthetic oil in the mix as long as at least half the oil is castor. Fox Superfuel (which can be ordered from Fox Manufacturing in quart containers) is 5% nitro, and 29% castor. Sig Mfg makes a good all castor fuel with 25% castor. You can run a synthetic mix but one lean run can cause damage in the older style motors. That said even cool power and omega fuels thatare very low in oil contentare okay in aFoxif you add at least 8 oz of castor oil to a quart of their fuel and avoid lean engine runs.There should be a visible cloud of oil coming out the exaust when the engine is running... the smoke trail when flying on a calm night is impressive.Castor by not burning pulls heat from the engine and if you do get a lean run it thickens casing the motor to slow down without being badly damaged. Most synthetics will vaporize at high temps leaving very little to lubricate the motor. Synthetics also remove the glaze that forms on the iron piston in old style engines... that glaze helps protect the moving parts. An old Fox may have great compression and with one lean run on all synthetic fuel loose compression so badly as to be hard to start.
Bob