140 weight gear oil in W124 rear differential?
 

The manufacturer's recommendation for W124 cars is SAE 90.

If I put 140 weight gear oil in my W124, am I causing damage to the differential, because the oil is too thick to penetrate into the tiny gaps it has to?

What are car manufacturers' official positions on using higher-than-specified-weight gear oil in differentials?

If 140 weight gear oil (75W-140, 80W-140) is mixed with 90 weight oil (80W-90, 75W-90) by topping off, will it still provide good protection? Gear oils are designed to be mixing-compatible with other gear oils, but are different weight gear oils compatible too?

Do 140 weight gear oils really suppress differential noise much more than 90 weight gear oils?

Using heavy differential grease has been around for 80+ years..

The weight should be 85W-140 if you want to use a heavier weight.

Why is 85W-140 better than 75W-140 or 80W-140?

It's not. But if you set a "standard" then you need to select one. Otherwise it would be like Michael Keaton's reply in "Mr. Mom", "220, 221, whatever it takes".

Would someone post a picture showing, or describe, where the fill hole is on a w124 (87 300e specifically) differential. I'm only aware of the hole where the rear abs sensor is. Thanks,

M.B. DOC: What's a good gear oil weight and brand to masked the whining of my rear differential on my 97 S420? Would I have to worry on long road trips with this problem or should I just find a rebuilt?

Thanks, wlk1997

Any update on this?!

I'd be interested to know if the more viscous oil has successfully reduced diff / rear axle whine.

Are there any axle oil additives out there which are more 'respected' than most (they all claim to reduce noise, but do they do so noticeably?!)

Many thanks.

Mine has now Valvoline Durablend 85W-140.
The 140 oil made the differential a little less noisy than 90 oil.
Mine has really loud mechanical noise, not a mere whine. So no oil can eliminate the noise.
The differential is a little noisier on highway in hot summer than in other seasons.

As for additives, I considered Lucas because it is available locally. However, as Lucas 80W-90 was found to be corrosive in Amsoil's gear oil test (note: the test is not a third party test), I did not use any additive.

Thanks for the info, ktlimq.

I've used Mr. Moly gear lube additive, it is a MoS2 suspension, reduces friction. I haven't noticed any noise so I can't testify to any reduction.