Run out of Foley Hytork Fluid for your marine transmission? Don't use "gear oil"! Most marine transmissions won't tolerate SAE 90 weight gear oil. This heavy, bunker oil will damage the transmission's composite clutch pack. Additionally, the marine transmissions tight internal clearances, assembly tolerances, and roller bearings are just not designed for 90 weight oil. So, in the absence of our Foley Hytork Fluid use synthetic non-petroleum based automatic transmission fluid (i.e. ATF). We recommend Red Line Synthetic ATF because it significantly lowers heat build-up. Some of our field service vehicles have over 100,000 miles of hard driving on their automatic transmissions using synthetic fluid with no maintenance of any kind.