When discussing vehicle oil/lub additives, I think of them as being one of
three classes:
1)  Engine oil friction reducers.   I've found no compelling evidence any have
merit.
2)  Engine oil solvents, pre oil change cleaners and "flushes."  The jury is
out on these.
3)  Transmission lub additives.   I suspect some have merit.  Clearly huge
differences exist between manual transmission lubricants, and perhaps much of
this can be duplicated by additives.  In over-the-road truck tests, some
transmission lubs result in lower transmission temperatures, and that's an
obvious indication that they are lowering friction between mating surfaces.

Seal softening solvents are another issue, but it's clear that some leakers
have been improved greatly by them.
John