How much should I adjust torque specs for lubricated threads?

The factory manuals for motor vehicles give torque specs for most nuts & bolts. My (possibly incorrect) assumption is that these are for dry threads as would be used in a factory where no oils or grease are present at the assembly site, or where proper protection from these lubricants are provided.

As a home mechanic (or even a pro, working in a shop), it’s often the case that threaded holes will have some sort of lubrication in them due to leaks during running or drips/slop during reassembly. It’s also possible/likely that nuts or bolts will get some sort of lubricant on them as they’re being installed to hold a lubricated part in place.

Since lubrication on the threads will allow the bolt to spin more easily into the hole, it’s possible that torquing to the factory spec will actually over torque the bolt, possibly leading to catastrophic failure while in use. Is there some sort of standard amount by which a torque spec for a particular nut/bolt should be adjusted (reduced) to account for the lubricated threads? Would the adjustment be based on materials in use? (cast iron vs steel vs aluminum vs other)