1992 Cadillac Fleetwood lifter noise

Not a real believer in snake oils, if there is a problem, all the additives in the world won't correct it. Too much oil should not have been detrimental, unless it was enough that it was being aerated by the rotating crankshaft. Are you certain this is lifter noise, look in through the oil cap and check for oil out the visible pushrods. If you have worn bearings down low, the pressure loss may prevent oil from reaching the top end. If it is relatively dry at the top, you are not getting oil up there for some reason. You could have adequate pressure to keep the light off, but still have a problem of it reaching the top end.