Stash

Greetings,

Went on a 7500 mile trip after changing oil on our '06 E500 with Mobil 1 0-40. Had only 7200 miles on the car and put on 7200+ in three weeks. Used correct gas except one five gallon addition(to another five gallons of premium) of regular in Canada when there was no premium available. Did moderate grear braking in high mountains for about five days: nothing above 4000 rpm. Car began to smoke when started, clatter unmercifully at startup, and smell when especially when decelerating. Suspected all kinds of monsters including valve train, head gasket, blow-by, etc. Not a mechanic. My dealer put in Amsoil P.I. Performance Improver Concentrate and Voila, the symptoms have disappeared. Unless I really do smell something in the cabin from time to time. Dealer said it was carbon buildup. I hope this fixed it, but is the problem disguised? Will value your opinions.
Thanks,
Stash