Differential Oil Change

75W-90 weight gear oil is recommended for the differential, changed at 30,000 mile intervals. First, make sure you can loosen the oil filler plug (23 mm) and you have a proper hand operated pump for pumping gear oil. Then drain the oil by removing the lower plug (24 mm) and remove any metal contaminants bound to the magnetic insert on the drain plug.

Add a new washer (same size as the oil drain plug washer) and plug, then replace the oil through the filler plug. Aluminum crush washers are used on the engine oil, transmission, and differential oil drain plugs. All crush washers are the same size (part number 9956-41-800). Oil should be added to the level of the filler hole. Filling can be accomplished under the car using a hand pump.

Torque Specifications

Differential drain plug: 39-54 N/m, 4.0-5.5 kg/m, 29-40 lb/ft
Differential fill plug: 39-54 N/m, 4.0-5.5 kg/m, 29-49 lb/ft

Changing the Rear Differential vent

Detailed instruction is found on Miata Net.