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Joined: Jan 07 2016
Posts: 125

I am in the midst of doing diffs, T-case, and engine services.
I noticed ~after, of course~ that I was sold 75-140 full syn gear oil, and manual calls for 75-90........
155k miles on the beastie, will there be any issue with the wider viscosity range gear oil?
Did a search, and found no answers......(Did not even bother trying on the dark side!)
Cheers!
Tom

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Joined: Dec 30 2015
Posts: 4032

75-140 sounds a bit thick but I can't see it doing any harm. Might add a bit more drag to the system and cost you 0.001 less mile per gallon.......

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Joined: Feb 11 2018
Posts: 214

Personally I'd put in what's in the manual ;)

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Joined: Sep 14 2016
Posts: 426

Graph on page 5 of this link https://www.widman.biz/uploads/Transaxle_oil.pdf shows very little difference in dynamic viscosity for 75W-90 and 75W-140 over normal temperature ranges.

Graph and table here https://wiki.anton-paar.com/en/gear-oil/ for the fully synthetic version of 75W-140.

Page 10 of the first link https://www.widman.biz/uploads/Transaxle_oil.pdf shows a comparison between synthetic and ordinary 75W-90 dynamic viscosity. Synthetic is, as is often the case a little thinner.

However dynamic viscosity is probably not the best measure of what is actually happening in a differential as what happens to an oil film under load can be quite different between differently made oils. For sure the 75W-140 will be a better oil than what was originally specified. The wider range simply means that viscosity holds up better with increasing temperature. 75W specification is the viscosity at 0°F (-18°C), the 90 and 140 numbers give viscosity at 212°F (100°C) which are both thinner than 75 at 0°F (-18°C). Far more information that you want here https://www.substech.com/dokuwiki/doku.php?id=gear_oils , click the links at your own risk! You may miss dinner.

Frankly I'd not worry. Gear oil viscosities are much less than you'd expect from experience with single weight engine oils. Found that out way back giving my Norton Commander its first home service as the book called out SAE 140 gear oil for the gearbox. I was expecting to need a spoon but it just poured!

Clive

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Joined: Dec 30 2015
Posts: 4032

I put Millers 75W-90 fully synthetic in my rear diff and it made the whine I was hoping to cure even worse. Changed it for cheapo Comma 75W-90 mineral and it went quiet again. I seem to recall gear oil being rated at EP 90 with the EP standing for Extreme Pressure, presumably because it gets squashed between the gears?

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Joined: Aug 07 2019
Posts: 70

75 /140 grade oil is fine in the diffs , don't use it in manual gearboxes, they will not go in to gear

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Joined: Jun 17 2018
Posts: 259

in this case the numbers don't really denote thicker/thinner they relate to how thick the oil is at a certain temperature. A 75W-90 will be thinner at higher oil temperatures than a 75W-140. Both are the same thickness at lower oil temperatures. The 75W-140 is said to have a wider viscosity range - when more accurately it would be a wider temperature range. The idea of multi viscosity oils isn't to get thicker at higher temps - it's to maintain a said viscosity at the higher temps. Confusing? Yeah..