Need some advice. BMW guru Mike Miller uses Red Line 75w-90 in his vehicles, but I've read that the fluid doesn't last nearly as long as the BMW fill, which I understand is Castrol SAF-XO. Land Rover sells the SAF-XO oil as part # LRN7591 for $25 a quart at the dealership nearby or I can order it for $20 a quart online. With shipping, it's a wash on prices. What say ye, Red Line or LRN7591?

Are you talking about the rear diff? In my '02, I replaced the OEM fluid with Redline 75w-90 at around 38K miles. Then I did another change at 70K miles with Lubro Moly 75w-90. Haven't really noticed any difference with any of them.

Redline fluids protect, Im sure, but they never seemed to maintain feel from experience in my 91 318i.

Examples: 4.10 LSD in the car - always had a bit of a whine on the highway. Redline (75w-90 and w-140) would make it go away for a little bit, but it would slowly come back in time. Moving to Specialty Formulations diff fluid fixed it and it has never come back.

MT, specified for ATF fill. Redline MTF would feel real good at the start, but give it about a years use and it would get much harder to engage into first gear in the cold, and sitting with clutch in, shifter in gear, there was a lot of noise from the MT. Switching to Amsoil MTF 5w-30 type made it go away and has stayed far more consistent.

So Im not saying that redline stuff is bad, I just think it is geared towards track and enthusiast use where the miles wont be high and the change-out will be often.

Now, is XO or XJ the right diff fluid for your car?

I dont have an issue going with XO, but would also consider Amsoil SVG in the 90 or 110 versions (or if XJ is required, then 140), and would also consider M1 right off the shelf. We have done well with M1 and RP gear oils in various diffs that have very high (>200k) mileage.

I have always been impressed with the feel of Motul Gear 300 (75w90 GL4/5) in my Audi's manual trans. Also used it in the rear diff.

The 222 VI really contributed to this when it was cold, I would say. Factory trans fill was Castrol TAF-X which made things a bit notchy, and we had SAF-XO in back.

Yes, Quattro Pete, I'm talking about the rear diff. Here's a thread on a BMW forum http://www.bimmerfest.com/forums/showthread.php?t=587665

The word must have gotten around around the Land Rover gear oil because the price has doubled. Now the cheapest is $14 a liter at parts.com