Transmission fluid for 300E

[elau]

I pulled the dip stick for the transmission out today the first time since I bought the car and noticed the fluid seems low. There are two lines on the dip stick, and I assume that is the indicator where the fluid level should be. From what I can tell, my fluid level is below the line at the bottom of the stick and I think I need transmission fluid. Is my assumption correct? If I need transmission fluid, what grade should I be using? And can I mix what is already in there, being not knowing what kind is currently in the car. Or should I take it to a shop and have it done? If I can just add fluid, where do I put the transmission fluid in, right down the same tube where the dip stick is? Any help is appreciated.

87 W124
95 R129
98 W163