Gear Oil? Synthetic or not! Here we go again!
 

The oil story is one of those never-ending stories. And yes I did my homework and searched... and searched many of the posts are 2-3 years old and just wanted some fresh take on this subject.

My car's manual and the repair manual by Stu Ritter (for the W124 chassis)states that MB doesn't recommend Synthetic gear oil. Maybe that is just for older W124s. Mine is a 95 V8. I am about to replace my gear oil and picked up a couple of Mobil1 75W-90 Synthetic. It meets all MIL-GL5 requirements plus many more. Can I use this without any fear of any leaks or issues ( I've read somewhere that synth. tend to leak more than the regular oil), or shall I stick to what is in the manual??? Dealer wants $17/quart for the MB gear oil (not sure how much weight).

I've used Valvoline 85W-140 semi-synthetic in a 126 diff. for the past 75,000 miles. I live in a very warm climate - winter is non-existent. This viscosity may not be suited for others.

If the synthetic lube is not right for my MB, I've yet to see why.
Quite/smooth operation.

Wow 75W-140??
 

I though that was more for race cars!!! Where do you live Mike? I thought Texas was the hot hell hole here:p

I don't think I'll go above 90W unless someone has really good reasons for it? It does get a litlle cold here starting next month for 2-3 months especially at night and then back to the frying pan for the remainder of the year.

I was just wondering if Mobil 1 75W-90 is better than regular oil with 75W-90 or 80W-90.

I believe the Mobil1 75-90 is their standard syn-gear oil. I have switched over to this for my differential about a year ago. I am using this in a 201, not a 124, but all my materials urge against synthetics as well. Like you, I think that may very well be based upon old information.

I talked with a very reputable benz mech a couple months ago. His take was all the Amsoil and Mobil1 synthetics are of good quality and are okay to use. He did strongly urge against using synthetics in my auto transmission though.

I think the issue of causing leaks is not a yes/no issue. If a car has gummed up seals/gaskets from years of using poor quality fluids and infrequent changes, then yes a synthetic will cause leaking when you switch over. Synthetic fluids act as cleansers and with it take the crud that was often plugging the gummed gasket. If the car has always had regular fluid changes and decent fluids, switching to synthetics will only make things smoother and last longer.

I use Mobil 1 in the engine, transmission and differential of my 300E. I'm paranoid about things like this but I have no qualms about using true synthetics in the entire drive train.

mobil 1 synth is fine. One caveat, if your diff is a limited slip get a limited slip additive for it as well as the mobil 1.
m

I believe the 75, 80, 90 weight range is preferred for most normal driving as opposed to the 85W140, which I believe is formulated for those that race or tow. There's pros and cons to each type. You don't want to use 85W140 for every application, since it has modifiers which break down faster than the base oil, plus the modifiers displace a fraction of the base oil, which is where the lubrication comes from. But you DO want to use it for extreme heat applications, such as racing or towing.

Many Asian SUVs have 85W-140 listed in the factory shop manual as the std. refill lube. Nothing mentioned about extreme conditions or temp.

Not trying to compare older 126's or other MBs to Asian SUVs - merely pointing out that this lube is not that uncommon.

It's served me well in the 126 for the past 6 years.