I have not tried Seafoam, but I do add Marvel Mystery Oil to my gas at nearly every fillup with great results. 25,000 miles and I've not had a single problem with my '07 Silverado. I use 2 ounces at each full tank. My plugs have stayed in great shape, start-up and idle is never a problem and I get a pretty rock-solid honest 37-40 mpg, even with two-up. This spring I added 4 ounces to my crankcase before going out and running hard for 10 miles or so. The oil had been changed right before winter, and I had put about 1000 miles on it since the last change. (I just wanted to get any condensation and acids out) It was a dark-honey color before I added the MMO
. Afterwards, it was a LaBrea tarpit black, so there must be something to it's cleaning abilities. I changed the oil and filter after the run with the MMO, and the bike (still) runs great. One thing I noticed was that my clutch was slipping sometimes at high rpm
's and after the cleaning with the MMO, it has disappeared. Maybe it just needed some of the old clutch dirt cleaned out? MMO is cheap (a gallon is about $14.00 at WalMart) and I use it in everything I run....