[335dlci]

Just ordered some red line diff fluid. Didn't realize it's 75-140 until I got it. Oops. Is 140 too thick?

[lnxguy]

From Bob is the Oil Guy.

1/10 of an mpg lost to the thicker gear oil.
Longer lasting 'end with the thicker gear oil.
A couple HP lost to the thicker gear oil.

[OmahaDZL]

BB_cuda has some info on this very subject.

[BB_cuda]

Per Autotech (Parent company to Wavetrac), I'm using 75W-90 Motul Gear 300. I can't say whether 140 is too thick though. My gear oil selection was more aimed at keeping the new LSD happy. I'm not going to argue with the BITOG guys. They can get really passionate. I run 75W-140 synthetic in my truck's diff because that's what Mopar says to do.

FWIW, the fluid level in our diff's doesn't go real high in the case. Sort of like about 1/3 the height of the whole ring rear diameter. Truck's amount of ring gear submersion is alot more. My point is the pinion is not under the fluid level (when car is not operating) as its at the center of the ring gear[EDIT: I need to verify this statement]. BMW may have done looked into minimizing drag on the ring gear versus wear on the ring/pinion gears. To be clear, the pinion is getting bathed in oil from carry over from the ring when ring is spinning. I need to go verify my statement of pinion location relative the bottom of fill plug. The pinion is a stubby fat thing so the bottom hypoid gears could be getting a fresh bath as they go by.

Lastly, very early in my diff process (before i decided to go the Wavetrac route), i just went to dealer and asked for proper lube for the diff. They sold me a liter of BMW mumble mumble SAV stuff. I recall it had 75W-140 printed on it. I ended up returning it as i had decided to change directions and the new lube was going to be a waste as the lube was going to be drained in less than 1000 miles. Omaha has that diff now.

A certain someone i know is in process of removing cover so could i ask them to compare the fill plug's bottom edge to the proximity of the pinion gear.