The question of extended oil changes can be a contentious topic. Ask about them and you'll receive all manner of answers. Some of the common ones are often:
My father has been an SAE Certified mechanic for 275 years, and he changes his oil every 12 minutes, so I'm going to listen to him because his knowledge and experience trumps everything.
The manufacturer knows more than you do about what is best for their engine, so I stick to what is recommended in the Service Manual.
Oil changes are cheap insurance!
I change my oil when it gets dark.
You'll void your warranty!!!
Now, that last one is actually somewhat sage advice, because if something catastrophic does happen to your engine, having proof that you changed your oil "on time" and at the factory-recommended intervals might mean the difference between a warranty repair, or a $7,000 out-of-pocket repair. The rest of those arguments, however, aren't great. Oil color is not an indicator of remaining useful oil life. Oil changes might be cheap insurance, but I don't subscribe to the idea of needlessly wasting oil (or my own time) on unnecessary oil changes. The manufacturer does indeed know more than all of us on what a good "conservative interval" is that will cover the majority of their users' use cases... but it's just that; a conservative estimate meant to cover a wide range of use cases. The right interval for each vehicle can vary drastically given how it is driven, where it is driven, etc. And lastly, yes, your father's 70+ years of experience definitely counts for something... but modern engines, and modern oils, are entirely different animals from the crude small-block V8s and very basic oils of yesteryear. Experience can be a good thing, but past experience does not always translate to best practices in today's world.
I decided to risk my warranty in the name of science, and while nobody should make decisions based on one person's personal experience, I figured I could share my results here as one more data point for folks.
Oil: AMSOIL 0W-20 Signature Series Synthetic
Why this oil? Because it seemed well-loved by many folks, and because they specifically advertise extended intervals in their literature.
Filter: OEM Subaru
Yes, I know AMSOIL sells extended interval filters as well, but to hedge my bets a bit, I decided to continue using the OEM filter and replacing it at shorter intervals. On the FA20, the oil filter sits right on top of the engine, so swapping filters is a 60 second job.
Intervals: 10,000 miles for the oil and 5,000 miles for the filter
The factory-recommended interval is 6,000. I settled on 10,000 for several reasons: it's a nice round number, I autocross the car, I daily drive in congested Chicago, and this is my winter daily. Also, at the end of the day this is a Subaru engine, so I'm not trying to push my luck too hard :P
So, without further ado, I present all of my oil change test results!
1st Oil Change: March 7th, 2015 @ 5,989 miles
I did not do a "break-in oil change" as some folks do. I simply went straight from the showroom to the 1st recommended interval of 6,000 miles, at which point I swapped in my preferred oil. I did not perform an Oil Analysis at this first oil change.
Until noted otherwise, the use case for this vehicle for all subsequent oil changes is: daily driver/commuter in congested Chicago traffic, autocross on various weekends, daily driver in winter months (occasional ice racing), and some general hooning around on back roads/in the snow. Also the car got a header and tune at 67,000 miles.
2nd Oil Change: September 5th, 2015 @ 16,740 miles
3rd Oil Change: March 25th, 2016 @ 26,174 miles
Note: My not needing to add any oil over those 10,000 miles will become a regular theme with this engine despite the reputation of Subaru engines to burn oil. There will be some intervals where I did top it up with a fraction of a quart, but even past 100,000 miles there are 10k mile intervals where no oil was needed.
4th Oil Change: October 8th, 2016 @ 36,012 miles
Note: For what it's worth, starting with this oil change, I now tow a 700 lb tire trailer with me to autocross events. Not that it makes any substantial difference in oil wear, but just figured it was worth mentioning.
5th Oil Change: April 8th, 2017 @ 46,350 miles
6th Oil Change: January 21st, 2018 @ 54,373 miles
Note: The question marks were because I didn't remember if I did or didn't add any oil, but suspected I might have added some and so just listed half a quart because I wasn't sure if it mattered any for their analysis.
7th Oil Change: October 10th, 2018 @ 64,579 miles
8th Oil Change: July 13th, 2019 @ 74,280 miles
Note: Here's where things got a bit interesting. At 67,000 miles, I installed an ACE A-350 EL Header, which necessitated a re-tune of the vehicle. My first tuner seemingly struggled to get the car to run properly. I had a lot of issues with hesitation, maybe some pinging and pre-detonation. It just wasn't happy. I suspect this caused a little bit of added wear on the engine, which showed up on this oil change in the form of higher levels of chromium and iron.
9th Oil Change: November 28th, 2019 @ 79,402 miles
Note: So after seeing the numbers on the previous oil change, I decided to halve my interval and check on things after only 5,000 miles. I also eventually gave up on the original tuner, and had my car re-tuned by the legendary Zach Tucker at CounterSpace Garage. This oil interval was after all of the tuning on his end, and the results were much lower chromium and iron levels. I still decided to do another 5,000 mile interval change after this one just to see how it looked without all of the mayhem of tune revisions and whatnot.
10th Oil Change: May 2nd, 2020 @ 84,937 miles
Note: Another 5,000 mile oil change just to see what things looked like.
11th Oil Change: November 11th, 2020 @ 94,999 miles
Note: Back up to 10,000 mile intervals! I should note I had my first EVER Check Engine Light with this car during this interval. At around 91,000 miles, I got a P000B indication ("B" Camshaft Position Slow Response). This was solved by replacing that VVT Solenoid, which cost me $75 for the part, and took me 3 minutes to replace as it's easily accessible from under the hood.
12th Oil Change: November 5th, 2021 @ 104,601 miles
Note: 100,000 miles and only 1/3 of a quart of oil added over 10,000 miles (which includes autocross). I'd say we're doing well!
13th Oil Change: September 27th, 2022 @ 114,707 miles
Note: No oil added at all!
14th Oil Change: October 22nd, 2023 @ 121,043 miles
Note: So this oil change happened much sooner because I suddenly own two more vehicles, and thus the BRZ doesn't get driven as much (in the interest of keeping it for a lot longer, I've decided to retire it from winter duty before it starts rusting apart.) So I decided to do an oil change at 1 year to see how things were. The wear metals didn't drop much, but this is also the first season where the car was co-driven at almost every event (I usually don't do co-drives) and so while yes, autocross isn't a track day, that's still twice as much hard driving as she usually sees. Since everything still looks good, I may try extending my oil change intervals based on time rather than distance at this point. The next oil change may not be for 18 months, for example. We shall see how many miles get put on her, and what I decide to do.