I run a 04 Merc 50 hp ELPTO and have had good luck with the Pennzoil XLF syn. blend. Also ran about 1 gal of the Walmart stuff through the motor when I first got it (to use up what I had from another boat that I had just sold). Didn't notice a difference when I switched, but the syn blend should help some with carbon buildup, etc. I also add fuel stabilizer every 3-4 tank full, and before storage. Like first2fish said, I usually get a few gals in the spring when Bass Pro puts in on sale.