I drive a 1998 W208 series Mercedes-Benz 320 CLK that's had oil changes every 9000 miles and has been utterly reliable since new. The 3200cc M112 series engine is as smooth now as when I first got it; however, in the recent cold weather I noticed the automatic transmission struggled a bit climbing a slight rise out of the drive when in reverse. This meant I needed to add lot more power than normal to get her moving up the rise. Going forward is unaffected. I noticed this last winter too but not during the summer we've just enjoyed. Reverse on the flat seems ok and the sluggishness only happens after the car hasn't been used for a few days. When I asked Mercedes-Benz about changing the transmission oil a few years ago I was told the box was 'sealed for life' so the oil has never been changed. The oil level on the transmission dipstick looks fine and the fluid is clear with no burnt smell. With 129,000 miles on the clock do you think I should get a transmission oil change done regardless and will it make any difference?