I replaced the manual transmission fluid and differential oil in my 1998 BMW 323is, both of which hadn't been changed in at least 100,000 miles.  This fluid in the transmission and RP Max Gear 75W-90 in the differential.  I expected some improvement, but it runs much quieter and smoother than I thought it could.  Sometimes it is like I nudge the shift and it sucks it into the gear I want.  Really surprising how smooth it is.  And my car is quieter even though I never thought it was noisy before.  Now the main sound is the engine when accelerating and if I'm at cruising speed I just notice the quiet.Having said that, I should note that the existing transmission fluid was black and only about 1/4 to 1/3 full.  There was some form of slow leak since the transmission was caked with dirt and oil yet I never had stains where I park my car.  So I'm comparing to what was an obviously under-filled transmission with old fluid.  The first 50 miles or so after replacing the oil the shifting was notchy and terrible and I was thinking I'd have to put something else in.  I was also worried I'd fried it due to low fluid.  But then one day it shifted smoother, and the next even smoother, until it was noticeably better than before.  Now a week later I'm really impressed.Forgot to mention my transmission is one of the manual ones that take automatic transmission fluid (Getrag) and this is rated for that usage.  Also I've noticed I have to go to 2nd gear much sooner now because the car gets to speed much quicker.  So it seems to be responding better to the throttle as well.  That seemed to have happened right after the change but I was babying the car due to the rough shifting so I only really accelerated normally a few days ago.Update 06/2012 - Several tanks of gas later and my MPG is higher.  Really improved the highway MPG, and it just did 32.09 MPG, doing the relatively flat round trip Los Angeles to Palm Springs plus some other mainly highway driving. My car has been getting 23 to 24 for the last two years and maybe 25 to 26 before that.  I've only hit 32 mpg once before in August 2009, and have had only 6 tanks of gas get over 30 mpg in the almost 12 years I've owned the car.  Stop and go driving is just slightly higher than before though as the prior three tanks of gas were 23.03 mpg, 27.38 mpg, 24.99 mpg.Update 10/2012 - The mpg the last 3 tanks of start and stop local driving were all over 27 mpg. The EPA estimated mpg for my car was 17 city/ 25 highway. So it seems to be doing well given it has the original engine and transmission.