Engine Additives...Stop smoke/oil burn

I think he means a carbon stain from his exhaust pipe. If that is the case and it is
indeed black , I would suspect that you are running rich . (is it black or blue smoke??)

As for additives I can tell you that when I sold BMW's their engineers sent out a memo
on the use of them . It said "save your money , we have found no positive effects
from any of these so-called miracle additives. ie, slick 50 , energy release etc. "
It also outlined how teflon works and pointed out the melting temp of it ( hotter than
your engine will ever get) and how things like forks much less fast moving piston rings
tend to damage teflon
They did however state that there are benefits from good synthetics . Mobile One,
Redline, etc