If its a 5speed manual box type 717.411 its capacity is 1.5 litres of suitable MB ATF. yes thats ATF fluid not gearbox oil. As has been advised with the vehicle on level ground clean the area of dirt round the level/fill plug on the side of the box--can be difficult to get at---needs a long spanner. Be prepared for fluid to flow out right away if the fluid level is up to the mark. If not make up a sort of curved dipstick--bit like a shepherds crook out of wire the red section from a bit of 1.5mm twin and earth is ideal or any thick insulated wire. You can hook this into level /fill hole to test where the fluid level is. If you have to top up the box you may have to fill it via some form of flexible hose/squeezible container to "pump" the fluid in.
If not make up a sort of curved dipstick--bit like a shepherds crook out of wire the red section from a bit of 1.5mm twin and earth is ideal or any thick insulated wire. You can hook this into level /fill hole to test where the fluid level is.