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Before heading out on a 2+ week, 5000 road trip, I brought my 2016 Forester (50k miles at the time) in for new brakes, an oil change and an inspection for anything else that might need to be looked at. The inspection did not turn up anything. A couple hundred miles into the trip my gas mileage went from 27 steadily down to around 12. On a hunch, I filled with premium gas and that issue cleared right up. After a couple tanks of premium, returning to regular, MPG stayed at 27. Is it safe to assume I just needed to run some good stuff through it to clean out the fuel injectors? And that it was just bad timing that it happened on the trip?

No other problems with the car during the trip. I checked the oil on our last fill up before arriving home and all was good. A week or 2 later (after driving 200 miles tops since returning home), my engine oil light (low oil not oil pressure) came on and when I checked levels, it was bone dry, so I topped it off and called my mechanic. He told me that unless there's a check engine light on, which there isn't, there's not much he can do... but "bring it in anyway. we'll run some codes".

I know that some of that year's Foresters have had issues with burning oil, but this is the absolute first we've seen of it, which also struck me as weird.

My actual question is

Should I just get an oil change and run premium through the car every couple of tanks and skip the mechanic, saving myself a couple hundred bucks for him to tell me they couldn't find anything? Or despite his statement about the check engine light, might there be something he can find?

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I think what you ran into was a bad batch of gas, which was cleared up with the next fill up. Could have been water in the fuel or whatever ... who really knows. More than likely it wasn't the premium gas which fixed the poor gas mileage issue, it was just getting a fresh tank of gas. Premium isn't necessarily the "good stuff". It just has chemical additives which causes the fuel not to burn as easily. You could still get bad premium gas just like you could get a bad batch or regular. Realistically, you are more likely to get bad premium gas then you are to get bad regular, mainly because more cars use regular then premium, which means the premium sits around a lot more in the underground fuel tanks then does regular. This means it has a larger chance of collecting moisture than does regular, which is usually how gas goes bad.

As far as what your mechanic said, the CEL is a good indicator. If that's on, there's something wrong. If it's not on, there's usually nothing wrong, but who knows. The computer cannot account for everything, especially if it isn't monitored. That includes things like tires, bushings, leaks, or other damage. If you are unable to check the vehicle out for yourself, paying the money to have your mechanic do it for you is great peace of mind before a long road trip. Does that mean if you take it to him absolutely nothing will happen on the trip? Something absolutely can happen. It's just if there is an issue before you start, the mechanic can help you detect that which will help you stay off the side of the road. In my mind it would be money well spent.

As far as an oil change now goes, is it due for one? If not, don't worry about it. Just get it changed at the proper interval and you should be golden. If you aren't detecting anything wrong with the car after the road trip (ie: is it running differently then before you left?), then save your money and leave the mechanic's visit until the next trip.

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  • Thanks for the input. We're already back from the trip. The bad gas milage lasted for 3 or 4 tanks (that's when it occurred to try Premium). I've driven 5000+ miles with just Regular without any incident since then. Rather than filling with Premium gas, is it advisable to use a separate additive? Sounds like this might have been a fluke. The more concerning part is that the oil level went down far enough for the light to come on. It is time for an oil change so I'll do that at least. Thinking I'll skip the mechanic though.
    – Daveh0
    Commented Sep 28, 2022 at 14:47
  • Running a fuel system cleaner through will not hurt a thing. Products like BG44K or SeaFoam and not the STP in the small bottle. The small fuel treatments have their place, but do not work as well as a cleaner does. It sounds to me like it was a fluke, but something to be aware of going forward. Commented Sep 28, 2022 at 16:15

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