The gang over at rec.moto went into great detail about additives. check out
the info posted there by T.Coradeschi.
Also, if you have a sticky lifter, try a quart of auto trans fluid. I did when
it was suggested to me, and boy did it work. Not only that, but my oil has
remained cleaner looking since I changed it afterwards. It seems to have some
strong cleansers in it. 
C.