Changing The Transmission Oil on a Subaru Impreza WRX STI
Subaru recommends you replace the transmission and front differential oil every 30,000 miles.
The front differential and gear box share the same oil on the 6 speed manual in the STI.
Generally this is a pretty quick job once you've decided on an oil to use.
Get the car in the air
Jack up the front of the car, drive it up onto ramps, or if you're lucky enough to have access to a lift, put the car in the air.
Drain The Oil
Remove the transmission oil dip stick. It's located on the passenger side of the transmission near the turbo, under the intercooler. This should be within reach under the hood. Take note while your here, that you'll need a funnel or tubing that can reach down to this dipstick hole in order to fill the transmission back up.
Remove the transmission pan drain plug. This uses a 14mm hex socket. Most of the oil will come out of this hole. You can do either of the two plugs first, order doesn't matter.
Remove the 21mm front differential drain plug. Note that later years use a T-70 torx.
Clean the plugs and Drain Holes
The larger plug from the front differential has a magnet in it. Be sure to clean off all the particles that have accumulated. Pictured below is a dirty magnetic drain plug.
Be sure to wipe down the surfaces around the drain holes. Removing any dirt and grit from these surfaces will keep the threads clean and undamaged.
Re-install the drain plugs
Screw in the differential drain, tightening to 51.6 ft-lb.
Screw in the transmission pan drain plug, tighten to 32.5 ft-lb.
Fill with oil
Fill the oil using a long bendy funnel or a tube. Stick the end of the tube/funnel into the transmission dip stick hole, and pour in your oil.
Everything says the trans takes around 4.5 quarts of oil, however we found that it was over-filled with just 4.25 quarts.
Based on our experience, it's much easier to check and re-check the fill level then it is to remove some oil after over-filling.
Slow down when you've reached 3-4 quarts and check it's level periodically as you go.
- Transmission drain plug: 14mm at 32.5 ft-lb.
- Front Differential drain plug: 21mm at 51.6 ft-lb. (note that later years use a T-70 torx)