Which Gear Oil
 

Recently I've begun wondering about differential oil.
In 20 some years, the diff oil has never been checked or changed. ('83 300CD). The owners manual with the car says 90 weight oil, but nobody sells straight 90w, only eg, 85-w90 75-w* etc. Actually, NAPA will sell 5 gallon barrels of it, but not quarts.
What is typically used nowadays?

(Eg: I have always used in all my vehicles straight 30w motor oil as my climate doesnt need multi vis oil.)

-c-

The 85-90 weight oil should be fine for your differential.

Just swapped mine with 85-90, works fine so far!

I have no idea.

For my 300Ds, I have successfully used hypoid gear oil SAE 85W-90 in the differentials for about the last 30 years.

I stopped at an "Auto Zone" & got some 85w90, & while there noticed gals of Rotella motor oil cheaper than Walmart, but it was multi vis. Asked if they had any in the back. he went to look, came out, no, but we have a bunch of quarts of Castrol (sae) 30, we dont stock it anymore, you can have it for $1 a quart. Got 2 gallons for 8 bucks, , , ,

The updated MB Maintenance Manual from 1992 says 85W90 or 90W are both fine for the W123 differential.
See: https://www.benzworld.org/attachments/3501-rear-axle-oil-change-pdf.426816/

Here's what I ordered for my W126. 85W90 and MB approved.
https://www.amazon.com/gp/product/B004FPP36M/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1

H-townbenzoboy

Thanks!
-c-

Liqui Moly GL5 75W 90 Hypoid 5 oil is what you want.

Also, I forgot to ask, my book says it takes one quart oil, for topping off, does one fill till the oil runs out of the filler hole? Or just drain & replace?

Fill until it runs out.

And loosen the fill port first just in case it might be seized and you find yourself without an easy way to fill it.

Dkr.

Thanks dkr

edit: & you others too