From Foley Engines' web site:
Hurth Gears: Care and Maintenance
Maintaining a Hurth gear can be easy if you follow a few procedures. This Tech Tip, one of an ongoing series published by Foley Engines, discusses the care and maintenance of the Hurth marine gear.
The Hurth marine gear is quite forgiving of owners who use less than ideal lubrication fluids. Many owners use common automatic transmission fluid (ATF) while others rely on shop manuals that (incorrectly) recommend SAE 20 engine oil. In our experience neither is optional. We had a major oil company develop fluid that seems to work best. We market it as Foley Hytork Fluid and supply it in gallon and half gallon containers.
Whenever the engine is removed for service it is a good idea to replace the spring loaded damper plate that mounts between the gear and the flywheel. These clutch-like discs are important to the operation of your flywheel. Our shop people have seen many a gear in for total overhaul whose damper plate was neglected and rusted on to the flywheel. For boats with high duty cycles and/or multiple users, like launch operators or those who just want absolute peace of mind, we have severe duty damper plates. These are marketed under the name of the Foley Diamond Damper for a slight premium.
Like a heavy duty damper plate, adding a cooler to a Hurth will extend its life, particularly in situations where the gear is at the upper limit of its horsepower capacity. Hurth coolers are an easy add-on and we highly recommend them, especially in launch applications.
-- Edited by Keith at 06:04, 2008-03-27