when i work at the marina i used mercury mercruiser stuff call valvetaine. not to good on spelling . i think it is spelled rigth? i ran 13 to1 engine on the street and this stuff work geat use to buy it by the case and added it it to pump gas. when i went to merc school they said mercury tested all the stuff at the time and nothing was good enough to have the mercruiser name on it so they made this stuff. i do not know if they still make it or not