Not a full time tech but I have replaced ATF in an 80 300D, 91 300E, and 91 300SEL. Sounds like you need to replace yours. Not that hard but it can be messy. Don't forget to drain the torque converter. Using a screw driver turn the torque converter until you find the hex nut. But first drain the transmission fluid from the pan. Then take off the pan. The filter is easy to replace, you see it after you take off the pan. Don't over tighten the oil pan when you replace everything, you can split the gasket, I know from experience.
Maybe a FT tech will weigh in soon. Good luck.