The Legacy manual specifies changing the front and rear differential fluid every 30k miles or 2 years. This Instructable will demonstrate how to do this for a 1999 Subaru Legacy and should be similar for other years as well. I completed this maintenance on my car at the Menlo Park TechShop and appreciated the extra space and tools in the auto-bay (I made it at TechShop).
Step 1: Gather Tools and Supplies
The tools I used included:
17 mm and 19 mm sockets
fluid catch basin
rags and news paper
For supplies I used:
differential fluid (3 quarts of 80W-90)
new washers (not shown)
Step 2: Elevate the Car
Use the floor jack to lift the front and rear of your car, and place jack-stands at all four corners. Try to elevate the car so it is level - this will ensure proper fluid levels when refilling later.
I used the tow hooks to jack up the car and then placed the jack stands under the pinch welds at all four corners.
Note: In anticipation of having all four wheels off the ground, I broke the lug nuts loose on the wheels, lifted the car, and rotated the tires. The manual indicates switching the front to the rear and rear to front; keeping the left on the left and right on the right sides. If you do this, remember to re-tighten the lug nuts when you lower the car.
Step 3: Drain the Fluid From the FRONT Differentail
Before draining the old fluid, make sure you can find the dip-stick and know where to refill the fluid. On my '99 Legacy the dip-stick is between the air intake and the fire wall, on the passenger side (don't confuse this dip-stick with the transmission dip-stick - the differential dip-stick is shorter and only has one set of low/full lines). The new differential fluid will be poured down the tube for the dip-stick.
Now place the news papers and fluid catch pan under the differential. The differential is between the exhaust and the sub-frame. It has fins on the outside (again, don't get confused and drain the transmission, which has a pan with lots of bolts around the perimeter and is on the driver's side).
Use the 19 mm socket and breaker bar to loosen and remove the drain plug from the differential. This is the step where I used the kitty litter to soak up the oil I spilled all over the floor. It came out a lot faster than I was expecting, so be ready.
Step 4: Refill the FRONT Differential
First, clean off the drain plug. It has a magnet to catch any metal in the oil and some flakes are normal. Once the plug is clean, put it and the new washer back in the differential and use the torque wrench to tighten the drain plug to 44 Nm or 32.5 ft-lbs.
Now take the flexible tube and feed one end into the tube for the dip-stick. Put the funnel on the other end of the flexible tube and start pouring in the new fluid. Take your time on this step and check the fluid level often (with the dip-stick). If you put in too much fluid, you can drain some out from the bottom. I used over a quart (the spec. is 1.2 - 1.4 qt.) to get the fluid up to the "F".
Step 5: Drain the Fluid From the REAR Differential
Again, make sure you can fill up the differential by loosening the bolt on the fill hole first (at the top of the differential) with the 17 mm socket and breaker bar.
Remove the bolt on the drain hole and let all the fluid drain out.
Clean off the any metal on the magnet from the bolt removed from the drain.
Step 6: Refill the REAR Differential
Using the new crush washer, replace the drain plug (with the magnet) and torque the bolt to 44 Nm or 32.5 ft-lb.
At this point my plan was to use the flexible tube and funnel to refill the differential. I tried to position the bottle and tube so gravity would do the work, but I made little to no progress. Instead of the tube, I switched to the turkey baster to refill the fluid. This technique wasn't fast, but it got the job done. Notice I cut the first bottle (from the 1.2 qt front differential) down so I could have a nice source for the turkey baster. Add fluid to the differential until it starts pouring out the fill hole, and then replace the bolt and torque to spec (44 Nm or 32.5 ft-lb). I emptied the second bottle of fluid and used some of the third as well - the spec for the rear differential is 0.8 qt.
If you are wondering where the extra fluid went (1.2 + 0.8 should have been only 2 bottles), I spilled some fluid while trying to wrestle the tube and funnel before using the baster.
Now lower the car and drive*.
*REMINDER: if you rotated the tires in Step 2, make sure you re-tighten the lug nuts before driving.