Pro Custom oil doesn't have to penetrate. Just scuff the poly, then add the Pro Custom oil. Keep adding until you get the sheen you want or go all the way to glossy and rub it down with pumice stone, then rottenstone to get the sheen desired.
On some antique rifles I am replacing the wood, I put down a few coats of Pro Custom oil first, then orange shellac to get the proper color, the back to Pro Custom oil to finish up. Like WTF said, everyone has their preferences for oil. I use Pro Custom oil for hunting rifles but for high dollar rifles I use pure tung oil that has been polymerized so it drys fairly fast. I might not live long enough of unpolymerized tung oil to dry. I wouldn't use BLO if you were holding a gun on me.
Phil
