It's great to hear that MMO can clean the soot.
I experianced the same thing when I started using Lucas UCL, I always had a little soot at the bottom of the exhaust pipe, the top portion of the pipe was bare metal, you know how water condensates at the bottom of the exhaust tip and then runs out, the soot was in that place and would follow down on the chromed tip, I noticed that on many cars. Don't confuse this with cars that are burning a little oil, as the soot on these cars covers the hole pipe and is oily to the touch, I'am not sure if the exhaust would clean itself in those cases.
After 2 or 3 tankfulls it self cleaned and and stayed clean, nothing but bare metal, grat that MMO can do the same.