Though it is thinner than std 90 wt gear lube - I am a wholehearted BELIEVER in regular old ATF in all BMW gearboxes up to the mid 90s! Leaks a bit less than Redline MTL, shifts great when cold - and it's kept my gearbox in tip top shape over the last 120+K miles tht I've personally used it. (My transmission was a USED unit installed in 1981 according to the service records on Emily, so now it has about 200K on it +/-) - and it is one of the smoothest, sweetest gearboxes in any 02 I've driven. (maybe I just got lucky) I'll rev her to 7200 rpm - but Im OCD about shifting smoothly, fairly slowly**, and matching rpms/heel/toe.

**unless there's an E30 M3 next to me at a light! ;-P

Paul Wegweiser

Go back to ATF. It worked for you.

Any synthetic 90 is the same viscosity as a dino 90 at 100C, the rating temp. Sure it is thinner in the bottle as you look at it, that't it's advantage, it doesn't thicken as much at temps below 100C (and so easier shifting). If anyone wants more info, try searching the web for centistokes and SAE 90 and the values of 9-12cST should show up. ATF reads 7cTS at 100C on my spec sheets, about the same as a SAE 20 oil (thin as piss on a platter).

So I have trouble understanding where the "synthetics leaks" comes from (other than it cleaned the grunge out left by old Dino).

BTW my gearbox runs at 180F using synthetic (not Redline) during cross crountry driving. So a 100C rating temp for oils is right on.

I second Weg's call on the ATF. I put it in a five-speed that came out of a donar car that I pulled out the woods with come-a-longs and paid a whopping 15 bucks for(the whole car!). That trans is in my 69' and shifts like butter. I put MTL in the 4 speed before I removed it, and it actually worsened the shift feel and leaked more than the dino oil that was in there!
Larry

What about a 50/50 mix of standard gear oil and MTL? The car is only used for track use no street use at all. So most of the time it is getting a good workout.