old-sailor,
While I have not used Seafoam on my old A4 yet, it does seem to fit in the same niche as Marvel Mystery Oil so I would probably use it in the absence of MMO. They had Seafoam on the auto parts store I was in about a week ago right next to MMO in NY State.
I say use it but with the caveat that you use it according to the label's directions.
It is the "foam" part of the Seafoam name that makes me wonder and since I've never used it I wouldn't know.
Don Moyer recommends using MMO as an additive to the fuel and in some cases to the oil reservoir as well. MMO is an oil, about a 3W oil and is not a foam. MMO also claims to have rust/corrosion protection additives which sounds a lot like the claims of Seafoam.
I use MMO to fog our engine come winter layup; that is, I put several good squirts into each cylinder. I will repeat this procedure as I see fit during the season to help loosen the cylinder rings, valves etc. Seafoam sounds like it should do nearly the same thing in both the fuel and lubricating systems.
http://www.seafoamsales.com/how-to-u...treatment.html