jayseas,

If you are asking if these engines will run on no-nitro fuel, the answer is yes, but not as well as with nitro. Fox made some modified heads to use with FAI fuel. I do not know if they are still made nor which models were available.

If by "old" you mean with a lot of running time, all-castor (no synthetic) lube would maintain the castor varnish seal for best compression in those iron/steel engines.

The amount of nitro included is a personal preference for your situation thing. For the Fox .15, Duke Fox recommended his "Missile Mist" fuel which I think is 25% nitro. I like 15%-25% for my Fox .15's (any vintage). Thebaffle .19 likes 10%-25% nitro, I have not run my schnuerle ones enough to determine but they seem to like the same mix. The .35 stunt runs well on 5%-15%.

For lube, I like 20%-22% castor or castor/synthetic mix for the .15 and .19. The Fox .35 stunt is a "special" situation of maximum engine in a small case. It likes 25% 28% lube because much of the generated heat is carried off via the lube, especially when tightly coweled.

Remember that these are personal preferences, others may choose a different mix.

If one of these did not answer what you were asking...ask again.

George