The rear differential is a hypoid gear set which directs power from the driveshaft to the rear wheels.  The Impreza uses an open differential, but the car is still capable directing torque in a manner similar to an LSD by activating the brakes on a slipping wheel.

Subaru’s maintenance schedule suggests merely inspecting the fluid every 30K miles.  In fact, replacing the fluid is not called for throughout the 132K miles covered by the schedule.  However, the “Maintenance Schedule 2″ chart in the service manual calls for replacing the fluid every 15K miles if the car was used for repeated towing.  Interestingly, the owner’s manual forbids towing of any kind, though the XV with a very similar powertrain does have a tow rating.

I chose to follow the middle road, a happy medium between every 15K miles and never, and change every 30K.

Before starting, you’ll need the following:

-New plug gaskets (CVT only)

-A 1/2” ratchet or equivalent driver

-Sealant: anaerobic is best, Permatex Grey is good

-A way to pump fluid into the differential

I’ve seen writeups online where people have drilled through the trunk floor below the spare tire to gravity feed the differential.  I suppose it’s a valid method, but I’m inclined to avoid putting unnecessary holes in my car if at all possible.

There are any number of hand powered pumping solutions available at auto parts stores and online.  I chose to use an unused windshield washer pump I had laying around.

First, safely raise the rear of the car to allow access to the rear differential.

Break the top plug loose before attempting to remove the lower plug.  If something bad happens at this point, you’ll be able to drive the car to a shop.

Next slide a drain pan underneath and remove the lower plug.  The plug has a magnet mounted to it, and mine had a fair amount of shavings.  Clean these off with a rag and some brake cleaner.

After the differential has drained, spray brake cleaner on a rag and use it to clean the oil from the threads of the drain port.

Install the lower plug into the differential.

Left: Shavings on the plug
Right: The cleaned plug

Next add 0.8 quarts of fluid to the differential, with a hand pump, powered pump, or through a hole in your trunk.  I chose Amsoil 75w90 Severe Gear and I believe it’s one of the best fluids out there, though there are many good ones, and some bad.

My pumping rig

The pump wasn’t designed for continuous duty, so I powered it for a few seconds at a time, giving it a chance to cool off.

Eventually, oil will begin running out of the fill hole.  At this point, the differential is filled to capacity and it’s time to install the plug.

Wipe the differential clean of any oil.  This will help you detect leaks in the future.

Lower the car, and clean up any spilled fluids and you’re done.  Happy motoring!