I've been mixing both in the fuel I use in my lawn mowers and ATV. I know FP Plus says it's a fuel stabilizer, but I remember a couple of years ago I used FP60 only in some gas in my ATV and it turned bad with a distinct varnish smell pretty quickly. It forced me to pull the fuel tank and fuel pump to unstick the check valve. This was after Hurricane Ivan and may have been ---- gas to start with. However, I've been adding both lately and wanted to get some opinions on whether these two additives are competing or complementary chemistries.