What year? Which transmission? If you have a '80 or newer M46 or M47, the recommended oil is automatic transmission fluid Type F (DON'T use Dexron/Mercon) . To check the oil level, put the car on jack stands or ramps (safety first!!) Look at the driver's side of the tranny. There you'll see two plugs screwed into the side of the tranny. The lower is the drain plug, the upper is the fill plug. Clean the dirt away from the upper plug and unscrew the plug. NOTE: I recommend you use only a 6-point socket on this plug. Sometimes the plug will be frozen into the tranny and will be rounded off with a 12-point socket or, worse yet, a Crescent wrench. This is specially a problem with the aluminum case M47.

When you have the plug out, stick a finger into the opening and see if you can feel the oil. The oil should come up to the opening. Replace the fill plug.

If you decide sometime to drain and replace the oil, I can say I've had good results with Redline MTL oil. The overdrive unit seems to work better and the shifting is smoother. Expensive but you don't need much.