75W90 vs 80W90 in an oem 80W90 spec

[aquariuscsm]

Curious about rear diff gear oil specs. Is there ever a reason *not* to use a synthetic 75W90 in an oem 80W90 spec? Gonna change my rear diff oil (I have some Amsoil AGL 80W90 put up somewhere if I can find it). Any disadvantage whatsoever to using a good synthetic 75W90? What's everyone's opinion on what is the best over the counter synthetic GL5 75W90? And then there's also Valvoline Durablend 80W90.

I searched all over for this info and all I read was a bunch of hunches and opinions but no actual facts. Thanks in advance and my apologies as I'm sure this topic has been beat to death

 

[bullwinkle]

I actually am interested in the answer too, considering RP 75W90 Full Synthetic in the Grand Marquis in my sig, factory fill is 80w90.

 

[aquariuscsm]

I just searched and found a used oil analysis of Castrol Syntec 75W90 and it looked great! I'll consider that lube till I figure out where I packed away my Amsoil.

 

[901Memphis]

As long as it meets all there's no cons especially if it's really cold where you live.

 

[SuperSalad]

Originally Posted By: aquariuscsm
Curious about rear diff gear oil specs. Is there ever a reason *not* to use a synthetic 75W90 in an oem 80W90 spec? Gonna change my rear diff oil (I have some Amsoil AGL 80W90 put up somewhere if I can find it). Any disadvantage whatsoever to using a good synthetic 75W90? What's everyone's opinion on what is the best over the counter synthetic GL5 75W90? And then there's also Valvoline Durablend 80W90.

I searched all over for this info and all I read was a bunch of hunches and opinions but no actual facts. Thanks in advance and my apologies as I'm sure this topic has been beat to death


A 75W-90 meets all of the exact same viscosity requirements that an 80W-90 does, so yes it is perfectly acceptable to use there.

Since the low temperature viscosity requirement is just a maximum of 150,000 cP at -26C for 80W, a 75W required to meet that very same maximum at -40C would definitely be under that same viscosity maximum at -26C.

 

[dave123]

For some apps might bump it up to 75w110

 

[Leo99]

I've been running 75W-90 in my Rav4 transaxle that calls for 85W90 gear oil for the past 25k miles. Doesn't seem to make a difference as far as I can tell.

 

[buck91]

This can become an interesting topic, a lot of controversy regarding conventional vs synthetic gear oil. I'm no expect but I do know a number of very reputable rear end specialists recommend conventional gear oil. Something to do with heat transfer at the molecular level resulting the the teeth faces running hotter with synthetic... I don't know.

I DO know that the only two times I've experienced SRA wheel bearing probables was with proper-weight Mobil 1 gear oil. Doesn't necessarily mean it caused it, just what was in there at the time.

 

[HTSS_TR]

My S2000 spec SAE-90 hypoid gear oil API-GL5, I used M1 75W90 for most of its life without problem.

 

[xxch4osxx]

Originally Posted By: buck91
This can become an interesting topic, a lot of controversy regarding conventional vs synthetic gear oil. I'm no expect but I do know a number of very reputable rear end specialists recommend conventional gear oil. Something to do with heat transfer at the molecular level resulting the the teeth faces running hotter with synthetic... I don't know.

I DO know that the only two times I've experienced SRA wheel bearing probables was with proper-weight Mobil 1 gear oil. Doesn't necessarily mean it caused it, just what was in there at the time.
That seems odd they recommend conventional for that reason. The big trucks come with synthetic from new and can go 500,000 miles before changing. If synthetic caused the problems those specialists claim, I think there would be many big truck differentials grenading.

 

[buck91]

Like I said, not really my area of expertise, just something I've come across again and again. Couldn't say about the truth or the reasoning.

 

[zeng]

Originally Posted By: buck91
This can become an interesting topic, a lot of controversy regarding conventional vs synthetic gear oil. I'm no expect but I do know a number of very reputable rear end specialists recommend conventional gear oil. Something to do with heat transfer at the molecular level resulting the the teeth faces running hotter with synthetic... I don't know

buck91, my 'hunch and opinion' on this was :-


Originally Posted By: zeng
Originally Posted By: Wampahoofus
I'd like to pick your collective brains. I picked up a Chrysler 300M with the 42LE automatic in it. It has a transmission and hypoid final drive in a common case, but with separate fluid reservoirs for each. The owners manual states specifically "use 80w-90 GL5 gear lube in the differential. Synthetic lubricants should be avoided."
The gear lube and ATF reservoirs are separated by 2 internal seals (if both fail, they can mix). I'm guessing the 'no synthetic lubes' edict comes from Chrysler believing that synthetic lubes are more likely to leak, and this is just a CYA move. Is there more to this that I'm not picking up on?

A mineral oil of 80W90 by 'nature', shall (post 2000's) have KV@40*C of 135-165 cSt and KV@100*C of 13.5-18.5 cSt. A synthetic "won't" be able to achieve this and it may not make marketing sense to the businesses.
For a synthetic to conform to SAE xxW90 gear oil grades ...... with primarily KV@100*C of 13.5-18.5 cSt which is non-negotiable in a way as far as SAE/ASTM are concerned, things were done resulting in say, a 75W90 oil grades whose KV@40*C would drops ........ way, way below minimum 135 cSt KV@40*C of typical 80W90 prevalent then.
I suspect some OEM manufacturers do realise this new formualation of say, 75W90 would result in drop in differential gear oil operating viscosity at an operating temperature they know best (which I would hazard a guess at 55*C-65*C depending on specific applications).
Not all OEM manufacturers are worried/bothered about this phenomenon.

But some OEM manufacturers do , realising detrimental effects of lower operating viscosity on their equipments ..... thus out go the so-called recommendation of "Synthetic lubricants should be avoided". This I call a conscientious manufacturer, ruled by engineers, NOT by a bunch of business beings.

https://bobistheoilguy.com/forums/ubbthreads.php/topics/4002535/Re:_Chrysler_42LE_-_avoid_synt#Post4002535

 

[artificialist]

In the places I worked, we would often get synthetic 75w90 to replace 80w90. Often it was how we could make a few more dollars at each repair.

Never once was it a problem.

It was also used where SAE90 was specified.

 

[901Memphis]

For us guys with the cold winters have you seen this demonstration of cold temperatures performance?

https://youtu.be/Y-FYdAzLjg8

 

[zeng]

Originally Posted By: 901Memphis
For us guys with the cold winters have you seen this demonstration of cold temperatures performance?
https://youtu.be/Y-FYdAzLjg8



Performance demonstration of an 80W90 at -40*C and -28*C is totally misleading .........
as a SAE 80Wxx is spec'ed for operation at temperatures above -26*C !

One doesn't demonstrate complete failure of gas fuel in a diesel engine test, do you?

Edit: Texas could've an 85Wxx if OP wishes.

 

[Shannow]

zeng,
that's the problem with the Esso cold start video and that one (which is actually a Mobil 75W90 synthetic versus dino FWIW).

In climates such as ours, the "wide spread" offers only disadvantages, but people are drawn to the conclusions that they "feel/posit" when watching how poorly lubricants used out of their design range perform.

It's certainly a pertinent video, but should be in context.

 

[zeng]

Originally Posted By: Shannow
In climates such as ours, the "wide spread" offers only disadvantages, but people are drawn to the conclusions that they "feel/posit" when watching how poorly lubricants used out of their design range perform.


Agree with you ...... in differentials ..... subjecting to super heavy duty as intended.

 

[CT8]

Originally Posted By: dave123
For some apps might bump it up to 75w110
Or 75W-140 syn.

 

[zeng]

Originally Posted By: CT8
Originally Posted By: dave123
For some apps might bump it up to 75w110
Or 75W-140 syn.



+1 ......
for higher MOFT film thickness and components protection.
The differential gears and bearings love it.
No, it won't be too thick .