I have long used Mobil One auto trans fluid. Whether it was due to being new fluid (several cars experience) or improved lubricity, they all seemed to have shifted smoother and faster.
Assume yours has a torque converter? Mine (123s and 126) have all had to be drained separately ... gotta spin engine to find the drain plug. Yours may be different.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg