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So I have an old westerbeke with what appears to be a hurth transmission. I changed the fluid on the motor and I am now looking to change the fluid on the tranny. Reading and doing research I believe it uses ATF fluid. My question is can I use ATF syntetic or will that make my seals leak.

:2 boat::2 boat::ship-captain::ship-captain::captain:
 

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My 15 yr old ZF hurth takes ATF and calls for old Dexron III. I only use those that are reverse compatible, which VI claims to be. Check you manual for the original spec.

Most sailboat transmissions have no filter, so changing the oil more frequently makes sense. The plates wear and putting that fine particulate back between the plates, causes even more wear.

Cheaper oil, changed more often, is always better than fancy oil changed less often.
 
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Whatever you do, do not use something labeled "Multi-vehicle formula." Those are blends that rarely actually meet the specs for any of the major brands.

Almost all of the current fluids (Dexron, Mercon, ATF+4) are backwards compatible. A few notable exceptions are 8/9/10 speed transmission, PHEV, and CVT fluids.

Switching to synthetic will not cause any leaks. What causes leaks is the detergents in new fluid breaking down gunk and build-up that has otherwise kept the seals from leaking.
 

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We changed to a synthetic ATF product (Amzoil) about a decade ago. No noted problems at all. Our Hurth model 50 had a reputation for being delicate, so we changed oil often. Never a problem.
Only caveat I have heard from an engine/trans distributor is not to mix the types - use all of one or the other. Other advice is not to extend the change interval. Change it just as often. Given that the transmission held so little oil, like about 10 oz, you are unlikely to run out...
:)

Which HUrth do you have?
 
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