Forester STI – Engine, Gearbox and Diff Oil Change

Whenever I get a car I tend to change all the oils early on so I know it’s all fresh and decent quality stuff. The Forester was no different so I got myself some Millers oils for the engine, gearbox and differentials and a genuine Subaru engine oil filter and sump washer.

For the engine some Millers Fully Synthetic CFS 10w50

For the Gearbox and Differential some Millers CRX 75w90

Genuine Subaru oil filter part number 15208AA100


The limited slip differentials in the FSTI are Suretrac items and do not require a special LSD diff oil. A manual transmission oil is all that is required, in particular a 75w90.

Engine oil first.


Plastic fixings coming out so the access panel for the oil filter and oil drain can be spun out of the way.

Like so.


Remove oil filter.


Clean mating surface.


Fill new filter with oil.


Fit new filter.


Sump drain plug.


Draining the old oil before refitting the sump plug with new washer.


New oil in the top.


If you read any guide anywhere about oil changing for a Subaru boxer engine you will most likely be told you should disconnect the crank position sensor and turn the engine over a few times on the starter motor before reconnecting the crank sensor. This is supposedly to prevent the the con rod big end bearing potentially running dry briefly on first start up and being damaged. Well I don’t know if I’m fully on board with the necessity for this step but it’s such an easy thing to do and the dreaded bottom end knock is not something I want to hear so I do it.

The sensor.


The sensor unplugged.


Now I turn the car over a few times on the starter motor. Then reconnect the sensor and start the engine as normal. This can cause a check engine light to show but turning the engine off and on a couple more times will clear the CEL.

Now the gearbox and front diff oil.

The gearbox and front diff take about 3.5ltr and the rear diff about 0.8ltr so a 5ltr bottle is enough to do both with some spare.

The front diff and gearbox share the same oil but they each have a drain. One requires a 14mm socket and the other a 21mm.


Both drain plugs.


Drain plugs removed.


Magnetic drain plug cleaned before refitting.


The gearbox can be refilled through the dipstick hole. The way I did this was to fit one funnel past the intercooler to sit in the dipstick hole then use another funnel to get oil to the first funnel.


You can see the red plastic funnel down there.


Lastly the rear differential.


As always with fill and drain plugs. Make sure you can remove the fill plug first! If for some reason the fill plug is stuck you don’t to find that out after you’ve drained the oil.

Both the drain and fill take a half inch drive.


Out with the old.


Magnet cleaned.


I used a large syringe to fill the diff. It’s not the quickest way but it’s the least messy and least wasteful of oil.

I always take a picture of the odometer reading when I service a car as a simple record. This is obviously a mix of km from it’s life in Japan and miles from it’s life here in the UK.


This was the first oil change I did on the Forester and according to the image info this was taken in October 2017 which goes to show how far behind I am with updates on the Forester as it’s now Occtober 2019!