I do not know if they could go as far as 0w-90, interesting question though. The problem with it would be how quickly it would shear down and drop out of grade. The larger the viscosity gap the more viscosity index improver is needed to bridge the gap and these can shear down easily in an engine let alone a diff. It is the same reason there is a common viscosity gap between certain grades, such as 0w-30/5w-30/0w-40/5w-40 then when you get to the SAE50 it kind of goes 5w-50/10w-50/15w--50/20w-50 up at the top end SAE60 you have 10w-60/15w-60/20w-60. They could make a 0w-60 I guess if they wanted but it would very soon be running as a 0w-40 or less as it shears down and stops doing a good job.

Cheers,

Guy

 

Much of the question comes down to "what's good enough?" In the engine 10W-30 oil temperature intuitively rises to 100°C much faster than than a 75W-90 gear oil will. I suspect the homework for this was done by the engineers designing the systems and needs no tweaking by we mere mortals for other than specialty applications.

-- Chuck

 

I understand multi-grade and am aware that the operating temp is the highest number whereas the lowest number is for cold temp "starts." However, I read somewhere that the wider the range, the more prone to sheer it is. Perhaps the individual didn't know what he was talking about. Regardless, the old 90W is the new 110W as has been mentioned before.

 

BTW, I was talking specifically about a grade that is considered relatively thin for the diff to start with if you're going to do track days. 75w90 vs. 80w90.

 

A multigrade will shear slightly quicker then a monograde, but then quality is also a key factor here is a well. A decent 75w-90 will out perform a poor monograde 90 and visa versa, the shearing of a 75w-90 in a diff will still not be enough to cause any issues after all 75w-90 is designed to do a better job than SAE90 overall.

The old SAE90 is not the new SAE110, if that was the case what is the current SAE90? It is still the old one. The viscosity range for SAE90 was effectively cut in half and the higher viscosity half became SAE110, but this is a non issue as according to what I have seen back then most SAE90 oils were not in that higher range of the viscosity bracket anyway. This is why no car manufacturers have retrospectively changed their diff recommendations to SAE110 or recommending it going forward because it was not the norm.

Cheers,

Guy

 

The problem with multigrade oils is, that the viscosity improvers shear down, that causes the oil to get much thinner over time in use.
Some shear down fast, some slow. In a gearbox, the stress for the viscoity improvers is much higher than in an engine.
Thats why a monograde oil like a SAE 90 is a good idea in a differential. It will last longer.
Nobody could say wich kind of viscosity improver the oil manufacturer uses. A virgin oil analysis also dont show it.
But for sure, the better viscosity improvers are more costly for the oil manufacturer.
Her is a PDF from RAVENOL with a little bit of Information about this:

https://www.ravenol.de/fileadmin/con...esentation.pdf

 

Just for sharing Information:
I discoverd two differential oils from PENRITE that carry GL6 (!) approval.

https://www.penriteoil.com.au/applic...-90-full-syn#/
(This one also carry the ZF TE-ML 19C approval, to my limited knowledge the most strict and demanding specification for differential oils.)


https://www.penriteoil.com.au/applic...f-oil-80w-90#/

 

The Castrol B373 also claims to be of GL6 performance https://www.opieoils.co.uk/p-712-cas...arbox-oil.aspx

 

GL6 has been obsolete for decades. They ran out of 1960's or '70's Oldsmobile engines needed to test the oil so I'm uncertain how it can be certified anymore. GL5 spec is still current.

-- Chuck

 

They used a Mk1 Mustang.