I think I was mixing my own, and I've forgotten what all was in it. It seems like universal pigments, linseed oil, sanding sealer, and some thinner.
One brush to put it on, then newspaper wad to soak most of it off, then a dry brush to pounce and move around to smooth it out, constantly wiping it out to keep just the right amount in the brush, without leaving any brush marks.
There is a bit of a learning curve to know just how much to wet out, wipe off, then work the rest while it's at the right viscosity before it gums up.
I'm not sure what the stuff they sell now has in it.