In this article, we will cover the steps required to change the fluid in a rear-wheel drive equipped vehicle.  Generally speaking, you do not have to change the differential oil on a front-wheel drive vehicle because it shares its fluids with the transmission.  This is an often over-looked service point for rear-wheel drive equipped vehicle - if your vehicle is RWD, it is likely time to change your differential oil.

Be sure to use the recommended oil for your vehicle's make/model - in our case, factory specification is 75w90 gear oil, so we opted to use Redline's 75w90 gear oil.

Start by warming up the vehicle's differential by driving it for at least 20 minutes. Once the differential is warmed up, park the vehicle is a level location.  If you need to raise the vehicle to access the differential drain and fill plugs, ensure the vehicle is leveled when raised.	[click for larger image] figure 1
[click for larger image] figure 2	Locate the differential drain plug and then place an appropriate oil catch basin beneath the drain plug. As a tip, to prevent any oil damage to your work surface, place a plastic sheet beneath the workspace prior to removing the drain plug.  Additionally, newspaper is ideal for catching any oil drips which may occur.
With the oil catch basin in place, remove the differential drain plug. Allow all differential fluid to drain. Caution:  The differential oil should (will) be very hot at this point.  Keep this in mind when you remove the drain plug.	[click for larger image] figure 3
[click for larger image] figure 4	While the differential oil drains, be sure to clean off the differential drain plug. As with the transmission drain plug, inspect the top side for large pieces of metal or other debris.  Generally, your differential will see some wear which will be indicated by a fine metallic paste which will be held by the magnet. Large metal deposits may indicate impending differential failure
Once all differential oil has been drained, replace the differential drain plug being sure to use a new crush washer and observe factory torque specification. Factory spec. for a Mazda MX-5 Miata is: torque: { 39-54 N/m \ 4.0-5.5 kg/m \ 29-39 lb/ft}	[click for larger image] figure 5
[click for larger image] figure 6	Next, remove and clean the differential fill plug.  Place a new crush washer on the fill plug to ensure a tight seal upon reinstallation.
Fill the differential with the approved differential oil. Factory spec. for a Mazda MX-5 Miata is:  specified oil:  { API GL-4 or GL5 \ SAE 90 } volume:  { 1.0 L \ 1.06 US qt. \ 0.88 Imp. qt. }	[click for larger image] figure 7
[click for larger image] figure 8	Once the differential has been filled, replace the differential fill plug being sure to observe the factory specified torque. Factory spec. for a Mazda MX-5 Miata is: torque: { 39-53 N/m \ 4.0-5.5 kg/m \ 29-39 lb/ft}
With the differential filled, you are complete.  Lower the vehicle and go for a quick test drive. Be sure to double-check the differential for any signs of leaking over the next few days - if the differential were to run dry, it will cause more problems than ignoring your regular differential fluid change schedule.