Jim I think by adding oil to your fuel does lower nitro content.That is why i was asking if i started with 25% all caster oil by adding enough castor oil to bring the nitro content into acceptable levels to run the fox 35.And yes theses engines are all old , 50's 60's 70's era.
So would 10% nitro be fine to run in all my Fox engines 15, 19, 35 with all castor?Problem is nobody around here sells all caster fuels except the all castor 1/2A fuel which is 25%nitro and 20% castor.I see sig sells fuel that is all castor 10% nitro. But it is like $45 a gallon and they don't sell quarts.I have heard and read that any synthetic oil in the fuel will burn away the seal that is created by the castor fuels, taking away compression, which would be no good.Does the fuel need to have methanol?

Here is a site i found. for blending fuel.

<font color="#ffb903" size="2">http://www.nitrorc.com/default2.asp?Introduction=http://www.nitrorc.com/fuelws</font><font size="2"> </font>