Maintenance for a Subaru WRX or STI 

The Maintenance Schedule

The maintenance schedule that counts is the one on Subaru’s owner site for that VIN: open the maintenance and service PDF after the car is selected. That PDF is more accurate than the booklet in the glovebox. When the dealer, the owner’s manual, and the owner portal disagree, follow the PDF.

The owner portal is for two jobs. It records dealer visits on its own, and it is a place to check recalls. Its schedule pages do not match the factory PDF. A notebook or a folder of receipts still works. Do not plan the next service from the portal’s intervals.

You can do the work. A factory shop manual from the dealer, or the paid download on Subaru’s tech-info site, is the procedure for that car. A Haynes or Chilton manual is the cheaper general version, and a job written for a Forester often still applies to an STI. The first time through a job, someone who has done it is worth more than a second guess at a torque.

Factory Fill Fluids

These are the fluids Subaru of America customer service named as the factory fill. A later viscosity in that car’s manual replaces the grade in this table for that car. It does not replace the table for every other car.

The manual-transmission oil used to be sold as Extra-S 75W-90. It is the same factory fluid under the High Performance name. Any 5W-30 changed on schedule is treated as the same oil. The brand on the bottle is a small difference next to the interval.

A non-STI rear differential left the factory on 85W-90. One early WRX manual prints 75W-90 for that housing instead. If the manual for the car in the bay prints a different grade, use that grade. An STI rear differential is not in that group.

What to Let a Shop Do

Print the PDF and hand it to the advisor. The invoice should match the required lines. “Recommended” lines are how a 30,000-mile visit gets expensive: a rear-diff service, a clutch flush, or anything else the PDF does not list. If the advisor will not take those lines off, the factory number is 1-800-SUBARU3. Ask which list is the maintenance schedule, the PDF or the shop’s recommended menu.

An independent or a tire-chain shop is often cheaper labor than the dealer. Bring the fluids. The shop will otherwise pour what is on the shelf. Ask for the empty bottles back so you can see what went in.

Add-ons to refuse

• Flush machines. The engine is built for a drain and a fill. Subaru of America has no position for or against a flush. An automatic is the awkward case: fluid stays in the torque converter and the cooler, and one estimate is that a drain exchanges about 35% of it. That still is not a reason to flush the engine.
• A dirty air filter. Between services the filter is supposed to look used. A used element catches more than a fresh one. The change point is the mileage on the PDF.
• Oil additives. Leave them out.

Transmission and Differential Oil

The 5-speed is picky. Factory 75W-90 is the fluid Subaru specifies. Other fluids earned a place because they calm synchro engagement, which is the one place a non-factory gear oil has a real benefit. A 6-speed will run on the factory fill. The same alternate fluids can help its synchros too.

The four-quart mix that shows up most often is not a factory spec. It is 1 quart of Red Line Lightweight Shockproof, 1 quart of Pennzoil or GM Synchromesh, and 2 quarts of Castrol Hypoy C 80W-90.

Lightweight Shockproof on its own is a 75W-140. Red Line’s application chart for this transaxle points at 75W-90 NS instead, and at 75W-90 GL-5 for the rear differential. The thicker fluid stays on the list for shift feel. The chart grade is the one to use when you want the viscosity the oil company prints for the gearbox.

Rear differential

A rear differential will run factory fluid or a synthetic without a compatibility problem. Most of them are fine on the factory fill, so a pricey synthetic is often money spent for no change you can feel. Some 2004 gearboxes, and some later ones, use a T-70 Torx on the transmission drain plug. A common Torx set stops short of that size. A single T-70 bit is the tool.

Power steering

Power steering fluid is not on the factory schedule. The intervals people use are about every two years, when the fluid goes dark, or about 50,000 miles. Putting it on the 60,000-mile visit is the compromise, and driving style is what moves that earlier or later. Use the fluid the owner’s manual names. A calm street car at 24,000 miles does not need it because a quick-lube lane said so.

Brake Fluid

Factory brake fluid is enough for most street driving. A big brake kit puts less stress on the fluid, not more, because the brakes themselves are doing more of the work. Hard mountain descents, autocross, and track days are the use that benefits from a better fluid. The first step up that people actually buy is Valvoline SynPower or ATE Blue. Past that, the gain has to be worth the price of the bottle.

Brake fluid is a consumable. Open a fresh bottle. Do not pour an old bottle back in. The exception is a leak or a low reservoir in about the first week after the fill: that is a leak hunt, not a reason to keep a half-used bottle on the shelf.

Moisture is the change point that ignores the calendar. A brake-fluid tester, a moisture meter, or test strips will read it. A digital reflectometer can read water in DOT 3 and DOT 4. At about 2–3% moisture, change the fluid, even if the last change was recent.

• Speed bleeder. A bleed nipple with a one-way valve. Open it and push fluid through. You do not close it after every pedal stroke.
• Vacuum bleeder. A hand pump on the nipple. You bleed at the caliper instead of pumping the pedal.

Used brake fluid, gear oil, and differential oil are taken with used motor oil in many places. Ask the local recycler before you combine them in one pan.

Belts, Sensors, and Filters

A factory belt is fine. The belts sold as NAPA are made by Gates, and Gates also makes the factory replacement, so the belt you want has Gates on it either way. A no-name belt howls. A Kevlar timing belt is an expensive option, and for about 99% of street cars the factory belt or an equivalent is not the part that fails.

With the timing cover off, the tensioner is the part to replace with the belt. Some people skip even that. The water pump, idlers, oil pump, and cam and crank seals are not worn out just because the cover is off. Replace one when you can see it leaking or worn. Replacing every part under the cover, because the cover is off, is the job people talk themselves into.

What the schedule often leaves off

These are judgment items. The factory PDF does not give them a mileage. The figures below are the working intervals, not a second official schedule.

Pull the PCV valve and shake it. It should click. No rattle means it is stuck and due, whatever the mileage. Factory and aftermarket PCV valves are treated as the same part. Buy the one you can get. The same is true of the fuel filter: factory or aftermarket, whichever is easy for that car.

Idle-air cleaning is for cars that still have a cable throttle, not drive-by-wire. The mass-airflow sensor is the other cleaning job. Both sit near the 30,000-mile service, and again before a tune. The front oxygen sensor usually announces itself with a check-engine light, or with fuel economy that falls off without a light. These cars are picky about that sensor. A factory sensor is the one that behaves. An aftermarket sensor can save the first purchase and cost a second one when it will not settle.

Fuel is already filtered at the refinery, at the pump, and by the fuel filter. A bottle of injector cleaner is optional. Spark plugs, engine oil, and coolant each have their own spec for that engine. The maintenance decision here is the interval and the fluid grade, not a second oil brand.

How Much Fluid to Add

Capacities are in the owner’s manual. They are not one number for every Impreza. The printed capacity is the worst case, the amount you might need, not the amount you should pour in one shot.

1. Read the capacity. Buy that much. Expect to have some left in the bottle.
2. Fill, then check. Use the dipstick or the level plug. Stop when that mark is right, even if the bottle is not empty.
3. Repeat. A second small pour after the level settles beats one long pour past the mark.
4. Look the next day. Check for leaks and top up. Do that again at the next fuel stop until you trust it.

A drain will not produce the full capacity. What stays in the housings is already in the factory number. Draining for days, running a flush, or chasing the last drops does not get you a better fill.

FAQ

Where do I find the Subaru maintenance schedule?

On Subaru’s owner site, in the maintenance PDF for that VIN. If the dealer, the glovebox booklet, and the owner portal disagree with it, the PDF wins.

Should I let the shop flush the fluids?

No. Drain and fill is what the engine is built for. The factory has no position on flush machines. An automatic still holds fluid in the converter after a drain.

What gear oil does a WRX 5-speed take?

Factory fill is Subaru 75W-90. The other choices on the short list are the four-quart mix, Lightweight Shockproof, and Motul 300, used because they help the synchros.

What fluid goes in an STI rear differential?

Subaru LSD 90-weight. Other rear differentials were filled with 85W-90, unless that car’s manual prints a different grade.

When should I change brake fluid?

When a tester reads about 2–3% moisture, even if the last change was recent. Street cars are fine on factory fluid. Hard mountain or track use is what justifies a better fluid.

Do I replace the water pump with the timing belt?

Only if it is worn once the cover is off. The tensioner is the part that goes with the belt. A Kevlar belt is not the upgrade a street car needs.

How do I know the PCV valve is bad?

Shake it. A good valve clicks. Silence means it is stuck. There is no factory mileage for it; about 30,000 miles is the working interval.

What bit fits the transmission drain plug?

Some 2004 and later transmissions use a T-70 Torx. A normal Torx set often does not include it.