Synthetic gear case oil

[Douglasdzaster]

Hello everyone,
I’m wanting to use synthetic oil for the lower unit on my outboard. It’s a 1992 Yamaha 40hp. The manual for the old motor says use hypoid sae 90. All the synthetics I’m finding are multi viscosity , 74w90 or 80w90. Would it be safe to switch to one of these?

 

[FishMan001]

80W90 is fine, but make sure it is for marine use, not automotive.
Quicksilver do a synthetic SAE90.

 

[Douglasdzaster]

80W90 is fine, but make sure it is for marine use, not automotive.
Quicksilver do a synthetic SAE90.

Thank you. I believe I found the Quick silver your speaking of.

 

[matt167]

80W90 is fine, but make sure it is for marine use, not automotive.
Quicksilver do a synthetic SAE90.

That only really matters if your LU leaks which it shouldn't be used if it does. The " Marine " gear oil's have an emulsifier for a little water in the gear oil.. Many gear oil's meet the marine specification if you contact the manufacture, and it does not say on the bottle.
It's not 'that' critical if you can't find marine gear lube locally

 

[Sea Rider]

As those marine grade gear oils are hard to find locally, been using standard automotive multigrade 80-90 gear oils since 1996 which were periodically changed after max 100 clocked hours with 0 issues whatsoever. Just be sure the gear case has 0 water intrusion...

Happy Boating​