Rear Differential 75w85 vs 75w90 - Any Benefit Either Way

[zeroptzero]

I just installed Redline 75w90 GL5 in the rear differential of my Subaru BRZ (torsen LSD), this fluid was easy to get quickly and Redline seems to have a good reputation for all gear oil, I was also super happy with their MT-LV in the manual tranny which worked extremely well.

The factory fluid in this car was 75w85 LX gear oil, I don't know if it would be synthetic or not, it had a reddish appearance. The drain bolt had a fair bit of black paste on it but the car has been breaking-in. Everything seems quiet so far, no change in sound/noise, I can't comment on fuel economy between the two fluids.

My question is if there be any discernable difference between 75w85 and 75w90 in terms of wear protection ? It has been a few years since I owned a rwd vehicle. When I had my Honda S2000 I found that rear diff wear decreased as viscosity was increased for fluids on that car, testing 75w90 , 75w110, 75w140 and SAE90 fluids over the years of ownership.

I suspect 75w85 is better for fuel economy, but wear protection qualities are more important to me. The car is only used in 3 season conditions - ambient temps 50-90 degrees F most of those seasons, no winter use. My concern is that the diff sump is very small , at just 1.3 quarts (approx.) , and bearing wear is most important to me. I don't know if these diffs rely on splash to get fluid flowing to the parts or if the important parts sit in the fluid ? The car will never see track use, just normal daily driving, mixed city/highway driving.

So is there much difference between 75w90 or 75w85 in terms of wear protection ? I believe the Redline 75w90 is 16 cst at 100 degs, and most 75w85's are in the 12-13 range.

Thanks for the replies. I know my decision has been made to run the 75w90 right now, but I the diff service is quite easy on these cars and I may be doing it more frequently than spec'd in the future. Thanks for any replies on this.

 

[edyvw]

75W85 is all about fuel economy.
Do you need 75W90? Probably not. But who cares? My Toyota recommends in rear and front diff. 75W85, but I run Motul 75W90.

 

[Jetronic]

You can expect wear to go down as viscosity goes up, like you already noticed.

What fuel consumption does is trickier, it'll go up but likely not at the same rate as viscosity did. when the loads on the rear differential are high enough to break through the oil film, friction goes up hard. And there's less of that with thicker fluid. But if you're not pushing the fluid near it's limit, thicker causes more heat and fuel consumption.

on a rear diff, I'd likely go up to xW-110 but if it's a transaxle xW-90 is where I'd go.

 

[ripcord]

Just for a frame of reference here's a chart that shows the relative viscosities of gear, motor and hydraulic oil. I've included a link to the original web page so you can see it at it's source.

https://www.nationwidefuels.co.uk/oil-guides/hydraulic-oil-facts-information/

 

[Chris Meutsch]

Don't worry about it.
My 4Runner is spec'd for 75w-85 in the diffs. I did that at 57k, but when I did it around 100k I switched to the MUCH easier to use Valvoline 75w-90 bags. I'm at 152k. Zero issues.

The only thing I stay true with is the straight 75w that goes into the transfer case. Ravenol for that. Not cheap, but they include a spout in the bottle so filling is easy!

 

[martinq]

I agree with you and thank you for sharing your info. A good synthetic 75w90 is what I would run (Amsoil, Red Line, HPL, etc).

Remember that the formulation is basically identical between a 80/85/90 GL5 gear oil so the EP wear protection should be similar once the gears are worn in. The viscosity will mostly affect drag / efficiency / heat / load capacity, and in your application I don't think it'll make much difference.

Do you have magnets on both drain plugs? That's the closest thing you have to a filter.

 

[zeroptzero]

75W85 is all about fuel economy.
Do you need 75W90? Probably not. But who cares? My Toyota recommends in rear and front diff. 75W85, but I run Motul 75W90.

Thank you . As this car is shared with Toyota I came across some Toyota gear fluids that were crazy expensive, Thankfully I don't have to use them

 

[zeroptzero]

You can expect wear to go down as viscosity goes up, like you already noticed.

What fuel consumption does is trickier, it'll go up but likely not at the same rate as viscosity did. when the loads on the rear differential are high enough to break through the oil film, friction goes up hard. And there's less of that with thicker fluid. But if you're not pushing the fluid near it's limit, thicker causes more heat and fuel consumption.

on a rear diff, I'd likely go up to xW-110 but if it's a transaxle xW-90 is where I'd go.

Thank you, I am also thinking with such a small sump size the overall effect on mechanical drivetrain losses is likely pretty small once everything is up to temp. It takes a very small amount of fluid.

 

[edyvw]

Thank you . As this car is shared with Toyota I came across some Toyota gear fluids that were crazy expensive, Thankfully I don't have to use them

Oh yeah. And they are nothing special. Redline, Motul, HPL, Amsoil. Delvac 75W90 is another really, really good stuff.

 

[zeroptzero]

Don't worry about it.
My 4Runner is spec'd for 75w-85 in the diffs. I did that at 57k, but when I did it around 100k I switched to the MUCH easier to use Valvoline 75w-90 bags. I'm at 152k. Zero issues.

The only thing I stay true with is the straight 75w that goes into the transfer case. Ravenol for that. Not cheap, but they include a spout in the bottle so filling is easy!

Hopefully you were able to avoid some of those crazy expensive Toyota fluids, I can't believe the price of some of them for diffs, trannies, and transfer cases, eeeek

 

[twX]

Subaru switched from 75W-90 to 75W-80 in the rear diffs on most their '18-'21 models, then they issued a TSB to switch back to 75W-90 due to worn diffs in some vehicles.

This bulletin provides a repair procedure for R152 type rear differentials. In some very rare cases,
a customer may experience a humming / high pitch sound coming from the undercarriage while
driving. This can be caused by worn differential gear surfaces resulting from hard usage of the
vehicle such as extreme weather conditions, severe road conditions, and / or steep incline driving.
A higher viscosity (thicker) gear oil has been introduced to provide additional protection against
premature gear surface wear under these unusual driving conditions.

This TSB doesn't apply to the BRZ, but it gives some insight into the conditions where a diff oil can be too thin (hot weather, rough roads, steep inclines).

For the 2022 BRZ, they seemed to have learned their lesson, and only reduced the grade from 75W-90 to 75W-85. Is this thick enough for severe driving conditions? Who knows. There's nothing wrong with using the thicker grade, especially on a summer car, though I doubt the grade is very important based on how you describe your driving.

 

[zeroptzero]

I agree with you and thank you for sharing your info. A good synthetic 75w90 is what I would run (Amsoil, Red Line, HPL, etc).

Remember that the formulation is basically identical between a 80/85/90 GL5 gear oil so the EP wear protection should be similar once the gears are worn in. The viscosity will mostly affect drag / efficiency / heat / load capacity, and in your application I don't think it'll make much difference.

Do you have magnets on both drain plugs? That's the closest thing you have to a filter.

Thanks, yeah my application will be pretty easy driving for the most part. As I noted the sump capacity is pretty small so it may not make a difference in terms of efficiency either way ?

The car has a large magnet on the drain bolt, it captures a lot of paste, no magnet on the fill bolt.

 

[martinq]

Thanks, yeah my application will be pretty easy driving for the most part. As I noted the sump capacity is pretty small so it may not make a difference in terms of efficiency either way ?

The car has a large magnet on the drain bolt, it captures a lot of paste, no magnet on the fill bolt.

I would look for a magnetic fill plug or modify the one you have, then you can compare and see how much metal is pulled from the fluid in the sump (drain plug) vs operating (turbulence / splash on the fill plug).

Don't forget to come back with pictures and an update!

 

[zeroptzero]

Subaru switched from 75W-90 to 75W-80 in the rear diffs on most their '18-'21 models, then they issued a TSB to switch back to 75W-90 due to worn diffs in some vehicles.

This bulletin provides a repair procedure for R152 type rear differentials. In some very rare cases,
a customer may experience a humming / high pitch sound coming from the undercarriage while
driving. This can be caused by worn differential gear surfaces resulting from hard usage of the
vehicle such as extreme weather conditions, severe road conditions, and / or steep incline driving.
A higher viscosity (thicker) gear oil has been introduced to provide additional protection against
premature gear surface wear under these unusual driving conditions.

This TSB doesn't apply to the BRZ, but it gives some insight into the conditions where a diff oil can be too thin (hot weather, rough roads, steep inclines).

For the 2022 BRZ, they seemed to have learned their lesson, and only reduced the grade from 75W-90 to 75W-85. Is this thick enough for severe driving conditions? Who knows. There's nothing wrong with using the thicker grade, especially on a summer car, though I doubt the grade is very important based on how you describe your driving.

That is good information, thanks for sharing it makes me feel better about my choice. It sounds like the bounce up to 75w85 was to reduce wear but still pushing the envelope on fuel economy. Glad I went with 75w90.

I have found it hard to figure out what fluids dealers are using on this car, the diff fluid has an LX designation and the tranny fluid LV, but when you look for Subaru fluids you don't come across this nomenclature . I think I am being better served by the aftermarket here. My use of Redline MT-LV has worked extremely well, very glad I switched to that fluid for my 6 MT.

 

[zeroptzero]

I would look for a magnetic fill plug or modify the one you have, then you can compare and see how much metal is pulled from the fluid in the sump (drain plug) vs operating (turbulence / splash on the fill plug).

Don't forget to come back with pictures and an update!

I could probably get a second drain plug that would fit the fill hole, they look to be the same size and threads. As long as the magnet doesn't interfere with any moving parts, it is about 1.5 inches long est ? I recall doing that on my Honda S2000. Will do on the pictures at the next drain OCI.

 

[johnmyster]

You can expect wear to go down as viscosity goes up, like you already noticed.

Another thickie. Is the BRZ gearset known to eat itself?

In that case more is better. 85w-140 all the things.

 

[Chris Meutsch]

Hopefully you were able to avoid some of those crazy expensive Toyota fluids, I can't believe the price of some of them for diffs, trannies, and transfer cases, eeeek

I reverted back to Toyota WS after using Idemitsu WS (which is probably identical). All other fluids have never been OEM other than that. Pink Zerex, Valvoline diff fluids, Ravenol transfer case. Havoline 6-quart box and a Supertech quart from WM.

 

[Garman19]

I reverted back to Toyota WS after using Idemitsu WS (which is probably identical). All other fluids have never been OEM other than that. Pink Zerex, Valvoline diff fluids, Ravenol transfer case. Havoline 6-quart box and a Supertech quart from WM.

I read Toyota ws is not fully synthetic, where as Idemitsu is fully synthetic, and they make the Toyota WS for Toyota

 

[SubieRubyRoo]

75W85 is all about fuel economy.
Do you need 75W90? Probably not. But who cares? My Toyota recommends in rear and front diff. 75W85, but I run Motul 75W90.

Plus the Gear 300 is extremely shear stable. I haven’t seen any KRL results for any 75w85s, but considering that only HPL and Motul were under 2% mechanical shear on 75w90, it’s hard to imagine there’s any 75w85s with better viscosity retention…

 

[thescreensavers]

From an OEM perspective and CAFE requirements any knob they can turn to improve MPG they will do. Fluid is an "easy one". They probably know that most people if they do maintenance at all would likely use a more conventional gear oil weight.