It's not like that, man.Saimsk, you're just setting yourself up for a long frustrating future... IMHO. When the solution to your stated gripe is so very simple... higher octane. Sometimes it is in one's own best interests to step back and get some perspective.
I already know before I own a vehicle that I'm going to use a higher grade of gasoline than what the "book" says to use. There is more than 60 years of driving behind that decision, most of those years having to do with the performance side of cars of one kind or another. Many cars, many years, and there has never been a car that didn't work best with a higher grade of gasoline. It's fundamental to how engines work.
Manufacturers' "recommended" is rarely the "best." It'll get you by. From the air in your tires to the gas you put in it to the care and maintenance... they give you a baseline of what to do, the MINIMUM you need to do. NOT the best thing you can do.
It's up to you to know the difference.
Maybe what's up with your discontent is something else... like not really liking the car or being sorry you bought it instead of something else you really want. In that case, and it happens all the time, cut the frustration out of your life and attack the real elephant in the room... get rid of the car asap and move on.
I put higher octane in my cars for one simple reason... it allows the engine to work at its best. I know that I've done everything I know to eliminate anything less than my engine's maximum without worry circumstances may combine to hurt it... in other words, suffer detonation or worse. To that same end I make certain it's otherwise taken care of as well, with clean filters and the best oil and all the rest. I wouldn't have decided to own it just to get the least from it. I want ALL my car has to give, for as long as I own it AND to have the most value left when I get rid of it.
No one nor no thing ever does its best or lasts as long by getting minimum care.
I love this car. The reason I'm so frustrated is because I love this car.
It's the best vehicle I could buy for the price in the segment with all the tech that I needed such as a large info screen with Android auto, eyesight active cruise control, dual zone climate control, alloy wheels etc etc. It's very comfortable and I think it is a very pretty looking car. But I'm just trying to cover my bases when it comes to any damage that has been done to the car as a result of the first service at subaru. Like I said, the knocking only started when I got the car back. It had the same 91 RON petrol in it when I handed it to them and when i got it back. And like I said, I don't mind using 95 or 98 RON in it. Just that I don't understand why it's started knocking all of a sudden. It's only a year old and I drive it 99% of the time on highways.
Plus, that's not the only problem since my service. There's also a strange whirring/whistling noise in the cabin when I'm coasting on the highway, noise goes away when I apply throttle. Comes back when I let go. Almost sounds like a field of crickets continuously making noise or like some kind of air leaking. It overpowers the cabin.
There's also a slight loss in fuel economy but I'm not too worried about that for now.
I plan to keep this car for the 300,000 km mark but with the current state of the car, I'm not very confident. Since it's still under warranty, I'd like to make the most of it and get what I paid for