Good point Jens. A iron/steel piston cylinder setup with lots of hours on it will sometimes get some of its compression from baked on castor on the piston. Apparently since most synthetics run so clean they will remove this varnish buildup and leave you with significantly lower compression. Duke Fox used to advertise that his Fox Superfuel (5% nitro, 29% castor) would put new life in an old engine (pre ABC era). If you have an old iron/steel with low compression, running 25% or so of all-castor fuel may improve performance.

Cox .049's run a bit hot and using all castor will usually necessitate cleaning varnish of the cylinder walls. This was often done with a gun cleaning solution containing nitro benzene. A wet brillo pad worked too.

I'm sure many already know this stuff, but some may not.

George