Yes, I have the same transmission and have used synthetic fluids for the last four years and 140k miles. Changing the transmission fluid to synthetic significantly improved the smoothness of shifts, especially when cold. When you drain the fluid from the pan you only remove about 3 quarts of the twelve quart capacity as most of the fluid is contained in the torque convertor. The first time I just drained, refilled, took the car for a five minute drive then repeated this four times to replace almost all of the old fluid.

I use Mobil 1 synthetic transmission fluid and replace the fluid every other oil change (3 quarts of fluid is cheap insurance). I replace the transmission filter every twelve months. If you do replace your fluid I would recommend that you replace your filter as well. It will give you a chance to find out if there are any deposits attached to the pan magnets.

Incidentally, I also use Mobil 1 synthetic differential fluid. I used Mobil 1 engine oil for a while but I drive an average of 800-1000 miles per week (my signficiant other, not me) so $30 for oil every three or four weeks was a little excessive. I use Castrol GTX, 20W50 in summer and 10W40 in winter. The car has 202k miles and does not use any oil.

Regards

Alex Lynch

I am considering changing to synthetic oil in my 735i's transmission. It's a 1987 model with the ZF4HP22EH gearbox. I was wondering if anyone else out there uses synthetic oil in their auto, and could offer me any advice, or let me know how it went in their car. Also if anyone has a specific brand they recommend. Many thanks.