As far as I know Sea Foam will not void the warranty. It was what was recommended to me as a winterizer for my 2000 Merc 200 EFI. I just make sure that I run it with my gas through the winter as a cleaner and stabilizer and have had no problems. It can be used on carbs,EFI and direct injected engines. Can be picked up at Wal-Mart for 7.54 a can. Sta-Bil is just that. A stabilizer. It keeps fuel fresh and takes care of any water in the gas, but doesn't have the cleaning agents of Sea Foam.