Differential Oil

I have 2006 Subaru Forester X. It has 47.5k miles. It has AWD with two differentials. Today I changed the rear differential oil and the front differential oil / manual transmission oil.

Magnets holding on to shards

The rear oil is a pretty simple process, you can access the differential case without using ramps. Make sure the car is level and the parking break is engaged. It uses a 13mm socket which is .512". Most ratchets are .5". You probably don't need to purchase the 13mm socket.Tools needed:

• 5" ratchet.
• oil pan
• differential oil. My Subaru takes about .8 of a quart.

Begin with warm oil. To change the oil, remove the top (fill) plug to relieve pressure, then remove the bottom (drain) plug. Let it drain. Examine the drain plug, it has a magnet on it that catches pieces of the differential gears. If you see large chunks then you may need to have it checked out by a pro, though changing the oil might be the preventative maintenance needed. Clean the plug and, once it is drained, screw it back in. Pour the new oil into the fill hole on the top.

This can be tricky because there is not a lot of vertical room in which to pour down. Some quarts have a mechanism to squeeze it out, others don't. It is full when the oil starts to spill out of the top fill plug, now screw in the top plug. Tighten both the top and bottom plug as tight as you can.

The front differential oil is shared with the manual transmission. If you have an automatic transmission it will have its own oil and you should not mix them. Tools you'll need:

• .5" ratchet.
• T70 Torx bit
• oil pan
• differential oil. My Subaru takes about 3.8 quarts
• tube and funnel
• clean cloth to wipe the dip stick

The front drain plug does not use the same 13mm socket, it uses a T70 Torx bit. I found mine at Advance, most auto shoppes carry them, it was $6. To drain, unscrew the T70 bit. This case has about 4 quarts of oil so be prepared for such a large amount of fluid. Be sure to check the plug for shards again and clean off the plug before replacing it. Once it is drained, screw it back in. 

The fill plug is on the top side of the same case, accessed through the hood.  It is in the center of the engine compartment all the way back against the firewall. It is a bit tricky to find it, but it has a yellow handle and is pretty deep in the back.  I have a very poor picture here, with the firewall to the left.

I recommend a tube and funnel, make sure the tube is all the way in the fill hole to so you won't spill any, differential oil is stinky. Fill it up, when you've emptied 3 quarts, begin fill slowly and stop to check the dip stick frequently. Differentials do not like being too full or too empty.

My thoughts on which kind of differential fluid: do not use a fluid marked for Limited Slip or LS. The additives in LS oil does not work well with the synchronizers in the Subaru manual transmissions. Many Subaru fora recommend a mix of Motul 300 and Redline oils. I don't like the idea of mixing oils as they will separate over night and they'll have to remix every time you start driving the car. YMMV but I feel find putting just the high end Motul in. Motul is difficult to find locally, I ordered mine from Annapolis Subaru, a vendor on SubaruForester.org. I got a slight discount. The range of prices for the dealership to change differential fluid is between $200 and $300. If I'm already doing this work myself then I can justify spending more on high end fluid. Especially for such a vital part of my car.

This maintenance greatly improves shifting. Shifting is smooth an much easier to get in to gear. Motul is rated for low temperatures so I'm hoping to glean that benefit on the -9F mornings here in Mass.

Doing my own car maintenance is very satisfying. It saves some money but I mostly enjoy having a deeper understanding of my car and how cars work in general.