Since we're on a Tohatsu kick and this weekend I'm replacing my water pump and gear lube, I want to ask. Yeah, I've read through tons of threads about lower unit lube and how it's different from regular automotive gear lube. I'm not a scientist, but I read the posts. Tohatsu, in both my owners manual and on their site, simply says "You can use any brand of lubricant that has a grade designation of API GL5 SAE 80 or 90 weight." Nothing at all about a marine specific lubricant. I've always used a synthetic 80w90 lower unit lube, but I'm wondering why. You'd think a manufacturer honoring a 5 year warranty would specify only the best (as indeed they do, for the powerhead lube). Just curious.
Lower unit lube... hear me out
- Thread starter whosmatt
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My 2 cents [worth every penny]. Lubrication for gears turning in a gearbox is a different animal than a rotating assembly of a combustion engine. Less heat, less energy forces, etc...and if water gets into your lower unit then it doesn't really matter if you have marine grade or not, you still have to repair and clean everything back to original. Marine grade grease for me was for parts that actually are expected to contact the water such as hub bearings on a boat trailer.
Fair enough, and I actually realized I've already asked this question once on the board. So please ignore.
But yeah, as far as the grease goes, I've lost a bearing buddy and didn't realize it until I dunked my trailer. Hub was fully packed with grease and the bearings nearly instantly rusted anyway. Salt water is the enemy.
But yeah, as far as the grease goes, I've lost a bearing buddy and didn't realize it until I dunked my trailer. Hub was fully packed with grease and the bearings nearly instantly rusted anyway. Salt water is the enemy.
Yeah, the thing about marine lower unit lubes is they come in convenient squeeze tubes that fit into the lower plug hole.
I would never say that the forces are less... You are putting the whole output of the power head through some 2 1/2 inch gears and one slip collar "clutch". The forces are pretty extreme. What keeps all this working is that they are water cooled so the lube never really heats up to much
I'd continue to use good lube. As good as you feel comfortable affording. The gears are expensive... The better lubes will mix with water and continue to lubricate pretty well. Long enough to get you home. Cheap oil will go to a nasty emulsion real quick...
I would never say that the forces are less... You are putting the whole output of the power head through some 2 1/2 inch gears and one slip collar "clutch". The forces are pretty extreme. What keeps all this working is that they are water cooled so the lube never really heats up to much
I'd continue to use good lube. As good as you feel comfortable affording. The gears are expensive... The better lubes will mix with water and continue to lubricate pretty well. Long enough to get you home. Cheap oil will go to a nasty emulsion real quick...
Both the lower unit and differential in a car are hypoid gears. So they both use a gear oil like a 75W90. But marine gear oil can handle water better. Seals do fail, fishing line wrapped around a prop shaft can cause a seal failure. So I myself always use Mercruiser premium. Its not a lot of money saved by using non marine. You can but Mercruiser lube at Walmart.
I look at it like an insurance package. If you totally destroy your lower drive by hitting something, then yeah, all bets are off. However, if you do develop a slow water leak and don't catch it in time, the marine grade lower unit oil will buy you some time instead of having to replace the assembly.
4wd
$50 site donor 2026
Have had leaks come and go on a couple outboard motors - one time the oil was milky - next time or two looked fine.
Used mainly Merc HP+ Gear Lube in 3 brands of motors ...
Used mainly Merc HP+ Gear Lube in 3 brands of motors ...
Use the marine grade. It won't foul as easy with water.
Tohatsu outboards are used all over in 3rd world countries. Marine grade lube is probably unavailable in Somalia or Costa Rica, hence the recommendation to use regular gear oil. Since you can readily buy Marine lube, but the marine lube.
Marine grade gear oils generally have many more rust and corrosion inhibitors than regular gear oils. They also contain vapor phase additives for the same. The lower unit is subject to much higher humidity than regular gear boxes.
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