I know I'm biased...but even if you don't run synthetic oil in your engine you should run synthetic lube in your differential regardless of tranny type.

1) It takes less than two quarts. So cost of product shouldn't be a huge issue.

2) With a good synthetic such as Amsoil Series 2000, the lube will last the life of the car (500,000 miles) easy. I change at 50,000-100,000 miles because I get the stuff wholesale and my wife loves the smell...but seriously there is no need to replace unless it gets contaminated with water, etc

3) It will make your rear end last much longer.

Use 75W-90 or 75W-140 (my favorite)

I use Mobil 1 85-90 for both regular and automatic locking rear axles. Check every so often and keep an eye on any leakage from the pinion seal, which 95% of the time is the cause of any leaks.

For limited slip rear ends, add an LS additive.

If you have more than 50k on your car its probably a good idea to change

it. If you have a limited slip you must use specific oil for that. If its

a regular open diff (most likely) then you need oil specified for GL5.

This is a regular maintenence item. Just not as often as engine oil. Don't

forget you should be changing your trans oil about every 15-20k mi.

Have a nice holiday.

Dave 82 242ti