Good point, but I'm gonna guess clean and dry. I never use oil or grease, can result in overtorquing (harder bolt pull for the same torque value), however on all threads going into my steel, aluminum, and titanium frames, I use anti-seize; The consistency of grease, but has fine metal powder in it to try to keep the coefficient of friction near dry, I'm sure it's more slippery than that, but better than grease and keeps threaded fasters and bottom brackets from locking in place. Also prevents galling, which can be more common on stainless steel, and I use stainless bolts for everything on the bike. A thread on here years ago, a mechanic swore by using oil on bolt threads, was certain about it, would not entertain talk of anything different.