I change my filter between 85 and 100 thousand miles ,, if i can i collect the fluid in the pan drain it through a
paint filter and put it back into the transmission I sometimes have to add 1 qt of transmission.
MOST transmissions do not work well with new fluid. after a filter change,, if u have too change the filter add the needed fluid
and drive. also use ONLY Mercedes transmission fluid (check for fluid used in slk320/ slk32/ c32 and c320)