OEM spec diff oil 85W90

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Jan 12, 2022
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Hello, I know this has been discussed but I cannot get a clear answer.
My 1988 Mercedes 420SEL calls for either SAE 90 or 85W90 for the differential. Not limited slip.
Can only find 85W90 from liqui molly but it is dino.

Closest to this weight I found is amsoil 80W90 synthetic. I live in florida so 75w-90 is probably not needed.

Any advice? I hear they say “new” 90wt oils are thinner than the old ones which concerns me for my 34 year old car.

Thank you in advance.
 
Correct me if i am wrong: They choose SAE 90 or 85W-90 back in the 80s because this oils contained zero VI improvers, therefore this oils would not get thinner (Sheared) over time in a differential. But the qualitiy of VI improvers has improved over the last decades, also the oils.
Oils like todays 75w-90 or 75w-140 are not likely to shear down over time like the oils in the 80s. And if you have douibts, just shorten the OCI for the Differential.

Yes, SAE 90 was divided into SAE 90 and SAE 110 in the 1990s. The gap in Viscosity of the old SAE 90 was huge, SAE 90 and SAE 110 divided it into a more meaningfull viscosity range. Also noteworthy is that many SAE 140 oils are just barely over the limit of the old SAE 90 / New SAE 110 range. For a older differential in a hot climate maybe worth a try.

What about Amsoil SAE 75w-110? Synthetic, no limited slip, and has a Mack approval. Would be my choice.
 
Thanks. So it’s true the new 90 weights may be thinner for my car. So ideally I should go with a 110 weight? I actually put a 75W-90 M1 a while ago but after reading about how thin it is it got me worried.
 
No, the new ones ar not to thin, its SAE 90, it is O.K. Your differential wil live forever with the 75w-90 M1.
But the 110 is on the thicker end of the older SAE 90 scale, it is likely to offer a little bit more protection. :)
 
Hello, I know this has been discussed but I cannot get a clear answer.
My 1988 Mercedes 420SEL calls for either SAE 90 or 85W90 for the differential. Not limited slip.
Can only find 85W90 from liqui molly but it is dino.

Closest to this weight I found is amsoil 80W90 synthetic. I live in florida so 75w-90 is probably not needed.

Any advice? I hear they say “new” 90wt oils are thinner than the old ones which concerns me for my 34 year old car.

Thank you in advance.
you can use any off the shelf GL5 80w90, changing the oil is far more important than using a synthetic
 
No, the new ones ar not to thin, its SAE 90, it is O.K. Your differential wil live forever with the 75w-90 M1.
But the 110 is on the thicker end of the older SAE 90 scale, it is likely to offer a little bit more protection. :)
Thank you. I wonder if the liqui Molly 85W-90 is a “thicker” 90.
 
SAE 90 old has a viscosity range of 13.5 - 24 cst at 100°c.
SAE 90 new is 13.5 - 18.5 and SAE 110 new is 18.5-24.

According to this PDF datasheet:


The Liqui Moly GL5 SAE 85w-90 is in the middle of the range with 17.5.
Hope that helps.
Appreciate it. now deciding between amsoil 75w-110 and the liqui molly. do you think the amsoil will be more superior due to being synthetic also? the amsoil has 20.9cst at 100C. i drove 8 hours at 85mph with my exitsting 75w-90 M1. hope no extra wear happened
 
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You are worriyng to much. But i understand it, you want the best for your car. Me too. :)
These cars where build for use at the Autobahn. It will drive forever with a 75w-90 at 85 MPH.
Use the Synthetic 75w-110 Amsoil, change it regullary and you have a oil that is more than enough.
 
You are worriyng to much. But i understand it, you want the best for your car. Me too. :)
These cars where build for use at the Autobahn. It will drive forever with a 75w-90 at 85 MPH.
Use the Synthetic 75w-110 Amsoil, change it regullary and you have a oil that is more than enough.
Thank you very much, I appreciate your advice!
 
Hello, I know this has been discussed but I cannot get a clear answer.
My 1988 Mercedes 420SEL calls for either SAE 90 or 85W90 for the differential. Not limited slip.
Can only find 85W90 from liqui molly but it is dino.

Closest to this weight I found is amsoil 80W90 synthetic. I live in florida so 75w-90 is probably not needed.

Any advice? I hear they say “new” 90wt oils are thinner than the old ones which concerns me for my 34 year old car.

Thank you in advance.
MB still advises 85-90 as of 2015.
 
You are worriyng to much. But i understand it, you want the best for your car. Me too. :)
These cars where build for use at the Autobahn. It will drive forever with a 75w-90 at 85 MPH.
Use the Synthetic 75w-110 Amsoil, change it regullary and you have a oil that is more than enough.
The liqui molly 85W-90 mineral oil has viscosity of 194 at 40C and the amsoil 75W-110 has 139.9.
Since I live in a hot climate (florida)... will this be an issue? of course the 100C is 17.5 vs 20.9
maybe this means the 85w will protect better while the car is warming up?
But then mineral oil usually is not as robust as synthetic...
and the viscosity improver index is much higher on the 110...
 
The LM is approved by Mercedes by name on spec 235.0, that means it’s up to the task and contains the right viscosity, protection, and additives for their differentials calling for 235.0 oil, it’s what I use as well in the CLK in my sig

It would get my vote, pour it in and motor happily for 50k miles before servicing it again, no need to overthink this, just use what Mercedes has tested and approved of for this specific application, IMO.

I change it every 50k or so and the diff is still dead silent and it always looks almost as good as the day I put it in each interval, I also used it in my old w124 E300 diesel
 
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