Since I am a new owner of a 2008 Smart, and never have used high test until now, is there differences between brands? A friend of the family who is a mechanic has told us that of all the engines he has rebuilt, the only engine that was ever clean inside was an engine that used Shell gasoline. I use Exxon or Sunoco in my cars since day one, BUT I want only the best for my 451. Any suggestions? I did fill it with Exxon 93 on first fill up. Thanks for you input.