75W140 or 85w140, anyone running it?
 

Anyone running these heavier oils in their drives? I know that Ford, and some other manufacturers, have specified 75w140 when towing heavy, in the rear diffs. I would think, that your typical sterndrive that is pushed pretty hard, whether it be due to HP, boat weight, prop style, etc, is a pretty severe load. I know the bravo drives that dont hold alot of fluid, can get pretty friggin hot. People think 75w90 is some super duper thick fluid, but when its at 200+ degrees its pretty watery thin.

I've been running your typical 80w90 in my SSM IV drives for the past few years since I've owned them. One drive was getting a little fuzz on the magnet at oil change. Thinking about going to the thicker oil. I can care less if it costs me a speed loss, IF it protects the gears from the severe loading/unloading (throttling) in a heavy boat with 800HP per side.

I use to run Synergyn 80w-140 Syngear II back early 2000's. Things I notice about using was - of course harder to pump in the outdrive and took a long time to drain when changes. Next the outdrive was quieter under load compared to Merc Hi perf gear oil. It also came out very clean and no material on the magnets.

A friend of mine who does oil analysis use to do all my oil analysis and the reports came out very well. I never had gear oil tested in anything over 575 hp. I will say this Mercruiser Gear oil High perf new and used samples were not impressive according to my friend. The first question he asked was, how much is the cost of their gear oil. After I gave him my cost (dealer) he laughed and said for the money they charge, you could do a lot better. I have not kept up with oil analysis service in years.

I run 75w110 Amsoil

I see the newer Ford trucks call for 75w140 in their diffs. My 2008 Grand Cherokee, calls for 75w140. My 1997 Chevy dually calls for 75w140. My Lincoln Town Car calls for 80w90, BUT, if it were a limo, with the same diff, they call for 75w140.

I cannot see any of those gears, being under as much stress as my boat drives, being held at full throttle for long periods with 800HP per side, in rough water.

You might be on to something. My '08 F150 calls for 75w140 synthetic gear lube in the rear axle. Do different oils affect operation of the cone clutch in the drive? Could there be any other drawback? (besides a little speed loss)

I don't have any clutches in my drives, but that seems like a good question.

I was shocked to see 75w140 specd on my jeep cherokee. Esp with how hard the car manufactures try to meet mpg concerns.

I know people hear "90" or "140" and they think it's sludge. But a 90 weight gear oil, isn't like a 90 weight engine oil. I'd think if my Jeeps diffs been working just fine with 75w140, in the extremely cold winters we get around here, for the past 150k miles , it should be ok in my drives that never see operation in subzero temps. ? Just thinking out loud here

With the bravo drives obvious heat problem (low fluid capacity, white chalky buildup, need for drive showers), is the 75w90 getting too thin at high temps?

We often debate the brand of oil used to our ears bleed. But nobody ever discusses the weight of the oil?

Great post mt
this could use some testing, do you gain heat with a thicker oil or after warm does it all even out plus the added protection

'YOUR OIL IS FLOWING TOO FAST!!!'

Good idea on slowing it down with thicker oil ..... = more cooling

FWIW, too thick or viscous of an oil actually retains thermal energy more so than a lighter, less viscous fluid. People forget oil is responsible for about 40% of cooling inside a typical engine for example(I realize we are speaking on gear lubes, however they act as a coolant as well). Granted some applications spec and demand more viscous fluids, however running thicker than called for has the thermal penalty. To add to this guys think adding capacity is also a good idea in every situation. Guess what, it's not! Larger than needed capacity merely takes longer to heat up, and once it reaches said upper temperature it takes a lot longer to cool back down due to decreased turnover. A situation where more isn't always better.

I hate to bring it up again, but NEO 75w-90 has proven itself time and time again. It's not only about viscosity here. That's only one piece of the puzzle. If a gear oil isn't shear resistant and doesn't offer a robust extreme pressure additive, it's not ending life anywhere near a 90wt oil while at 100 degrees Celsius! People forget about shearing which is always a present force in any gear box.

Many manufactures are specing drivetrain fluids with CAFE in mind. Fuel economy over the absolute best protection. There are always compromises. However, it's telling that some manufactures are calling for 140wt gear oils. My feeling is maybe it has something to do with their expectation of shear/load/maintenance of said gearbox... Building in a safety feature by merely selecting a thicker fluid makes a lot of sense( for example the used 140wt gear lube has a 90-100 wt at the end of life vs a 90wt which would/could shear down much more). However if you were to buy the best (OEMS are all about profit, so obviously they never do this), shear and lack of maintenance would be things rarely encountered.

And remember 90wt gear oil is measured at 100degrees celcius. The only way it "thins out" is if it shears down in grade. It's physical appearance and behavior(thinning with heat, thickening during cool down) with temperature mean about as much as the color of used motor oil, nothing! The only way you will know if it's doing its job and not shearing to an unacceptable level is to perform a used oil analysis. Film strength is a pretty strong barrier when it's in place. This is what separates searing hot metal, in a tiny space.