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I went to the auto parts store to get my fluids for my rockster and the only synthetic 75w140 they had was Mobil 1 with "LS" on the bottle. I am assuming I can use it, but am curious if there is an additive in it for limited slip that would not be a good idea to introduce to my transmission. Any advice??

Here is what the label says...

"Mobil 1 Synthetic Gear Lube LS 75W-140 Mobil 1 Synthetic Gear Lube
LS 75W-140

Designed to meet the requirements of light trucks and SUVs where an API GL-5 75W-140 gear oil is recommended.
Mobil 1 Synthetic Gear Lube LS 75W-140
Mobil 1 Synthetic Gear Lube LS 75W-140 is a premium performance, synthetic, multi-purpose, automotive gear lubricant designed to meet the highest level performance requirements of many modern passenger vehicles in all types of operating conditions including limited slip applications, as well as deliver outstanding power transfer performance. Compared to conventional hypoid gear lubricants, Mobil 1 Synthetic Gear Lube LS 75W-140 performs exceptionally over a wide range of temperatures.

Mobil 1 Synthetic Gear Lube LS 75W-140 achieves this through a unique proprietary combination of synthetic base oils and a unique additive system, including the SuperSyn anti-wear system, that deliver optimized viscosity-temperature properties together with the highest level of inherent formulation stability and protects against thermal and oxidative degradation, wear and corrosion, and viscosity loss associated with premature shearing.

Mobil 1 Synthetic Gear Lube 75W-140 LS contains special friction modifiers designed for limited slip-type differentials. In most applications, the addition of special LS (limited slip) additives is not required. For axles requiring the highest level of limited-slip performance, OEM specific LS additives can be added to this fluid.

For more information, see the Mobil 1 Synthetic Gear Lubricant LS 75W-140 Product Data Sheet and Material Safety Data Sheet."

I have done a bit of reading on this very subject and have come to the conclusion that in the Transmission 90wt or 80wt API class CL 5 to be the preferred lube, with 80w90, & 75w90 synthetic as acceptable substitutes, I believe 75w140 synthetic could also be used the transmission.

For the Final drive I have concluded that hypoid gear oil 90wt, API GL 5,is the favored, with 80w90 being a substitute, From my reading I wouldn't under any circumstances put synthetic in the final drive, NOW this is just my opinion and it is based only on what I have read (and that is a lot) mnn

mnnden wrote: From my reading I wouldn't under any circumstances put synthetic in the final drive, NOW this is just my opinion and it is based only on what I have read (and that is a lot) mnn

What is wrong with putting synthetic in the final drive? I just purchased 75w140 from the bmw dealer that is synthetic. They recommended it!

In the Final Drive the Manual calls for 90wt or 80w90 as an acceptable alternate, . ( I do know that BMW sells a synthetic 75w90 that states on the bottle it can be used in the final drive, but I don't think it is recommend in the manual)

The manual calls for 90wt in the transmission with 80w90 or 75w140 being a acceptable substitute

With the rate of Final drive failures in these bikes I aways wonder why anyone would use an out-of-spec oil in the final drive
I am in no position to argue with the dealer, but In all of my reading I can't remember anyone using 75w140 in the FD, (not that it isn't used, just I haven't read much about it) Being these bikes are prone to FD failure I would use what the manual calls for. As I said earlier this is just my opinion, based only on what I have read, There has been a lot written on this subject, mnn

From my experience with both heavy equipment / trucks and auto applications,
synthetics run cooler, are more fluid in cooler temps, and don't thin out as much
under load and under heat as dino oils.

I use 75W140 in the trans because the hot cat is right under it, and 75W90
in my FD. The somewhat less viscous 75W90 should flow into the small
spaces a little easier in the FD (i.e. bearings, etc) than the thicker 140.

When synthetic gear oils came out, heavy trucks that started using them
experienced cooler running trans and diffs and got at least 50% more life
out of them between rebuilds, and those gears & bearings experience
tremendous loads over long periods of time.

I hear ya macx, this oil thing with BMW's really goes over the top, I know of quite a few who do as you do, and have had no problems, in fact I have done it myself, but with all the FD failures I have decided to go by the book and use the 90wt, But I think the using 75w140 in the FD is a recipe for disaster, JMOP mnn

Final drive failures are not prevented by going by the book. It probably doesn't improve your odds either. FD failures predate use of synthetics, don't they?

Been using synthetic in all cavities for over 50k miles without issue. If you're going to get a FD failure, you're going to get one, and the odds are long in your favor. As long as it's the proper weight, I cannot see how a synthetic could increase the risk.
90wt FD and 75/140 in tranny--Mobil One
Usually Mobil One in the crankcase also: 5-10-15/50

Dr. Strangelove wrote:Final drive failures are not prevented by going by the book. It probably doesn't improve your odds either. FD failures predate use of synthetics, don't they?

Been using synthetic in all cavities for over 50k miles without issue. If you're going to get a FD failure, you're going to get one, and the odds are long in your favor. As long as it's the proper weight, I cannot see how a synthetic could increase the risk.
90wt FD and 75/140 in tranny--Mobil One
Usually Mobil One in the crankcase also: 5-10-15/50

As with many posts after a little dissuasion they take-off in a whole different direction, no said, or even hinted that synthetic oil was no good! The OP asked what anyone thought about using 75w140 in the transmission, and I simply stated that imop it was fine, I think (I haven't looked) that the manual says it is fine. This has nothing to do with the pros and cons of synthetics, I cautioned the op that IMOP I would not use 75w140 in the Final drive, ( I disagree with you in that using the wrong fluids in the final drive will increase your chances of failure, because it will!) As you stated "As long as it is the proper weight" and I don't think 75w140 is the proper weight for the FD (Synthetic has not a thing to do with it) I simply stated that I wouldn't use any synthetic in the FD because the Manual doesn't recommend it, NOT because it is synthetic, I am not much of a mechanic, and I don't pretend to be, but I do not know of anyone who uses 75/140 in the final drive. I do know people use 75/90 with no problems, (I did for awhile) but switched to what the manual recommended because I do think there are ways to prevent FD failure. I change fluids more often than what is called for and look for signs of failure, "an ounce of prevention is worth a pound of cure" what do you use in the final drive? I am sorry if anyone thought I was "Knocking" synthetics I sure didn't mean to, jmop mnn

Oh, no! this is why people shudder with an oil thread. And this is germane to the OP, I think.

No flame intended by any stretch, btw

as we both said, using the wrong weights of lube anywhere is asking for failure.

75/140 in FD is the wrong weight in the FD, but ok in Trans.

What I was mainly responding to was something that was implied, or maybe I just inferred; i.e., that "going by the book" in the case of lubricant for final drive would help stave off FD failure. My point was if the proper weight is employed and timeliness of exchange is observed, then synthetic vs not-synthetic does not matter. And whether you're talking of FD failure or clutch spline failure, it is a rare occurrence, and the cause is nebulous--gremlins, metallurgy, alignment, design, etc. Going by the book and using non synthetic, or straying from the book and using synthetic, neither is in any way a certain preventative.

Maybe we're saying the same thing?

I use synthetic because I am old enough to remember the, I think, original Mobil ONe commercials, in which they filled a car with Mobile one, drained it out, and ran the engine to the moon and back without damage, or something like that. It had that kind of sticking power. I figure if I am on some backroad in New Mexico or New Hampshire and somehow I get a tremendous leak of trans, or FD or crankcase oil, that with Mobil One I am more protected for a longer time. So I use it and am willing to pay the premium for that. I readily admit it may be wishful thinking, but I find it more comforting than going by the book-- in that particular case.

Happy Easter

John

Thanks John, "maybe we're saying the same thing" I think we are. mnn

I think I am agreeing with the guys that said the proper weight is more important that synthetic versus dino oil.

I used 75/140 for a while and had a FD bearing failure (not the whole thing). Have no clue if is was a contributing factor and make no such claim.

But I switched back to the recommended weight and dino version to boot! Since I am draining the oil at 6K anyway for inspection, synthetic is of limited value to me.

Note: I am not an oil expert, I do not play one on TV and I did not sleep at a Holiday Inn last night!

Thanks for all the input guys. I think i will be changing the FD fluid back to a 75w90 and continue to use 75w140 synthetic in the tranny.