If 6 speed, 3000 series transmission (like MD3060) 4th is direct, 5th and 6th are OD's. There will be a plate on the driver's side of the transmission with model and serial number-- good to have when ordering parts/filters as there are both shallow and deep pans.
Torque converter is locked in "high 2nd" and all higher gears. The lock up in second is at higher engine speed and feels like a mini-shift.
Servicing is pretty straightforward. Get the filter kit from an Allison dealer. It has filters, gaskets and instructions. Not a difficult job.
Then get a 5 gallon pail from the Allison dealer of Transynd 668 (the newest fluid that replaces earlier fluids).