I have a 2007 S-80 and have a transmission question. I just bought this used and it has 135k on it. when getting onto the freeway at around 50-60 mph, if I mash down hard the transmission seems like it starts bucking like it can't decide where to shift to. I have also noticed in going down a hill it may hold up into the gear it's it and actually slow down unless I accelerate then it shifts. I am not familiar with any quirks of a six speed trans and have always had Ford or toyotas.
Should I change the fluid? Or is it a software issue? The instruction book says the fluid does not need changing - kinda wierd.

 

Hi, if 135 K is miles and the oil never been changed then get ready for transmission change or overhaul.