Transmission fluid change interval

A gather that the recommended interval for a transmission fluid change is 3 years or 30k miles? And that is drain, change the filter if any, and refill, not flush
I ask because the users manual says nothing.
Does this apply to a CVT transmission?
What about differentials?

(2015 Forester, non-turbo, CVT)

Yes, on all counts.
CVTs need a fluid and filter change on the same schedule as “conventional” automatic transmissions, and I recommend that differentials be serviced on the same schedule.

In an effort to make their cars look as maintenance-free as possible, many (perhaps all) mfrs have dropped 3 yr/30k trans servicing from their maintenance lists. However, dropping this item from the list does not remove the reality that failure to do this maintenance inevitably leads to early transmission failure.

Transmission failure from lack of maintenance will not be likely to happen during the warranty period however, so the mfr gets the benefit of selling more…supposedly…“low-maintenance” vehicles, and the hapless owners get the bill for transmission overhaul.

I do have to admit that my last car got no transmission or diff. service in 120k miles, as per the manual, and I had no problems. Passat V6.

Question? does the CVT belt drive run in a fluid or in air surroundings? I had thought in air, but I may be wrong…


“does the CVT belt drive run in a fluid or in air surroundings? I had thought in air, but I may be wrong.”

Wrong on all counts.


The Subaru CVT doesn’t utilize a belt, and instead transfers power with a steel roller chain that–IIRC–is ~3 inches wide. Like all chains, it requires lubrication, so it is bathed in the CVT fluid.

For those who don’t drive a lot, I think 3 years may be a bit too frequent. My wife puts about 6500 miles on her Mazda per year; we will still change the transmission fluid at about 30,000 miles of course, which would be about 5 years.

A couple of years ago I read an informal study of transmission failures compared to frequency of fluid changes for a particular car line. I don’t have the text anymore, but one group of car owners serviced the transmission every 30K and the other group never serviced. At 150,000 miles the percentage of transmission failure was within 10% for both groups, which I didn’t think was statistically significant. So is there any real predictable benefit to servicing your transmission?

Eh, I think it can’t hurt. I still recommend fluid service and new filter where applicable at 30,000 mile services. Fluid is cheap, transmissions are expensive. Some customers agree, some don’t.

Subaru requires inspections really unless severe So you interpolate if your driving is one or the either. My guess few do severe while few also do normal too. Probably every 60k would suffice which interestingly is what my MDX comes up with and it actually monitors your driving habits/environment/style and even towing thru the vehicle computer to come up with an “answer”.

Most vehicle manufacturer’s recommend that the transmission fluid be serviced every 30,000 miles.

But they also recommend that the transmission fluid be serviced annually under the following conditions. If the vehicle is used for heavy towing, if the vehicle is driven at high speeds in hot weather, or if the vehicle is driven under high wind resistance conditions. Such as a heavy head wind or where a roof top storage container is mounted on the vehicle.


Most vehicle manufacturer's recommend that the transmission fluid be serviced every 30,000 miles.

Toyota is using the WS fluid…and they say it’s a lifetime fluid. I change it every 50k miles.

“serviced every 30,000 miles”, according to Subaru, that means checked, not replaced. For not severe use, that is.

However, if I interpret the Severe driving list strictly, everyone in New England falls under this regime. “basically any coast, country, or winter driving”.

I like Andrew’s interpolation, every 60k miles, replace the fluids, and am tending in that direction.


Change intervals depend on use and how much fluid is in the transmission. My big GM trans has 14 quarts or so of fluid, about 9 I can get on a fluid and filter change. I tow with it and at its first change at 80K the fluid still looked and smelled good. Now 40K later it is due for another because of the fluid I couldn’t get out.

The wife’s car only holds 5.5 quarts, never tows, has NO filter with a really nice drain plug and a change is just over 3 quarts. Did it at 60K (semi-burnt smelling fluid) and every 30K since. Wish I’d started it at 30K because its so easy.

A CVT, I’d do every 30K just because I’m nervous about the technology.

Differentials, at 500 miles if it has a limited slip (and so you can switch to synthetic) and every 50K thereafter.

3yr/30k is too often for me (unless the owners manual says otherwise). More like 5yr/60k.

Critical for your Forester: I would ONLY have it changed at the dealer, major problems could happen if wrong fluid used, the CVT will be very particular.

Mustangman, makes sense also.

Any other items that Subaru neglects? I noticed that antifreeze is also inspect only…


I am curious haw the CVT fluid is checked, you can’t see it unless you drain it from what I have been able to tell so far. There are no service manuals and the new Subaru’s are not on All Data or other services, you have to go to Subaru for a subscription, and it ain’t cheap.

But if you tow, the fluid exchange interval is around 25k miles.

There are just some things that you will have to accept, even if the manufacture does not list it as a concern.
I’d stick with 30K filter and fluid changes

Like the “Lifetime ungreasable bearings” that never last a lifetime. Sometimes you just have to follow your own instincts.


I have two Toyotas with the “sealed” transmissions, using WS Lifetime fluid. Two dealers and a long time independent transmission shop all said wait till 100K miles before changing the fluid.

“lifetime antifreeze” . . . that is what leads to plugged heater cores

Every 5 years is pretty reasonable for most vehicles nowadays

OK I am somewhat thoughtful, changed front and rear diff at 160kmiles, did not look too bad, trans fluid, 130k as it was getting a little dark, complete flush by disconnecting at radiator pumping into a bucket while adding 12 quarts more fluid. Planning on at least 200k inside.

I am curious haw the CVT fluid is checked, you can't see it unless you drain it from what I have been able to tell so far.

Our Toyota’s/Lexus have sealed transmissions (NON CVT). No way t check the fluid unless you drain it.

@MikeInNH With all due respect, are you 100% certain that your transmission is sealed?

The reason I’m asking . . .

My brother’s Toyota supposedly . . . according to the owner’s manual . . . has a sealed transmission

The book makes absolutely no mention of any dipstick

yet it has a dipstick, and a drainplug on the pan

For what it’s worth, you can’t really see the dipstick, until you remove the engine cover, and even then, it’s pretty far down

I personally don’t like the term “sealed transmission” because it often leads to poor maintenance habits