OS recommendation keeps you on the safe side concerning the percentage of oil. There wouldn't be that much difference between 17 and 18 percent. Many FS manufacturers tell you to NOT use 4 stroke fuel. This is because 4 stroke fuel has a lower oil content than 2 cycle, which gives more power... but a lean run could easily ruin an engine. If you're careful you can usually get by with the four stroke fuel, but I don't use it in my 91 surpass.