85w90 vs 75w90

[BlakeB]

I'm using 75w90 on Toyota's specifying 75w85. No issues, so far. I don't think you'd have any issues with 75w85 in place of 75w90. They did a study one time and I believe they said using a 75w90 in place of 80w90 resulted in about a 1% efficiency gain. I don't think the gains you'd see using a 75w85 in place of 75w90 would be significant or measurable on a small scale.

 

[Impatient]

What KV40 specs have you seen? I'm grappling with gear oil decision myself, and think there is a wide range of specs within the 75w90 offerings. For instance, Motul 300 has KV40 of 72.6, whereas Delvac has 120, and I'm not sure this is the highest (Amsoil Longlife has 129). I found a Chevron 75w85 with 72, so that KV40 is v. close to the Motul 300, while the KV100 is much lower (as you might expect). Meanwhile Motylgear (semi-syn?) 75w85 has 82 KV40. Also, compare Amsoil SeverGear to their LongLife, and you'll see overlap/inconsistency on the KV40/KV100 specs.

By the way, some of this depends on which Subaru, which transmission, which transaxle. You may need to be careful if you have certain manual transmission. There's the whole friction modifier/no friction modifier discussion. 2007 Subaru with Torsen rear specs 75w90, without friction modifiers. I didn't know how "thin" the Motul 300 was at 40c when I put it in my rear diff, but seems reasonably OK. It's possible a higher KV40 would work better. Motul 300 has a very "extreme" VII...so maybe it's a good 75w85 surrogate.

 

[Railrust]

I used 75w90 in place of 75w85 in my Lexus for five years (over 100,000 miles of driving) without one single issue or drop in fuel economy. And I'm doing it now again in my 2018 Silverado (40,000 miles), with once again no difference in performance or fuel economy. So I would think switching from 75w90 to 75w85 wouldn't be a problem...question is, why would you want to?

 

[dippschtick]

I'm running Ravenol 75w85 in my 4Runner but since it came with 75w85 I have nothing to compare.

I have had zero issues with the Ravenol though.

That being said, I'll probably go with a 75w90 on the next change.

 

[Gebo]

Originally Posted by jqgz
The Kv 100 isn't that far off but the KV 40 is a lot lower. I would think this would help gas mileage and responsiveness but am concerned about wear. Anyone have experience to share? I have a 2015 Subaru.


I think you will be amazed at how the viscosities among the 75W-90's vary among different brands of gear oils. Mobil, Amsoil, Valvoline, Schaeffer, Lucas, RL all have differing viscosities even though they are all 75W-90's.

 

[cronk]

When I worked at the local Nissan dealer, If a customer wanted synthetic fluid in the diff, I would get 75w90 Valvoline synthetic from the parts store up the street. Always worked well, never any issues.
As a side note, the rear diffs on many of the Nissan trucks had a little metal vent that had a tendancy to clog up with mud, rust or road debris.
Might want to check and replace if needed. A clogged vent will allow pressure to build and cause seal leaks.

 

[BlakeB]

Originally Posted by BlakeB
I'm using 75w90 on Toyota's specifying 75w85. No issues, so far. I don't think you'd have any issues with 75w85 in place of 75w90. They did a study one time and I believe they said using a 75w90 in place of 80w90 resulted in about a 1% efficiency gain. I don't think the gains you'd see using a 75w85 in place of 75w90 would be significant or measurable on a small scale.

https://apps.dtic.mil/dtic/tr/fulltext/u2/1039630.pdf