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You are taking the mickey!

We're paying $11 a gallon in the UK for gas!

Seriously, it's nothing to do with number of cylinders - your mechanic is absolutely wrong. The octane rating required by an engine is determined by a number of factors, but certainly not size or cylinders - more usually compression ratio & timing.

If Mercedes recommend premium, use it: It's worth the extra to be sure.

Most trbocharged engines, such as Porsche & Saab units, have APC control, which means that the engine will vary it's settings to burn fuel of any octane.

Your Merc isn't so clever, so it doesn't suit the engine to use low octane fuel - and if you do, you'll certainly get fewer miles to the gallon.

In the UK, your car would run far better as our 'regular' is 95 octane, and our 'premium' is 99.

If you're using a low enough octane that your engine 'pinks' or 'pings', you could be damaging it.

"one mechanic I had said that since it is only 4 cylinders it's not actually going to hurt the car to use regular"

Never go to that mechanic again! What an idiot! The number of cylinders has NOTHING to do with the fuel octane requirements!!!!

Try it. With today's computerized engines, probably the worst thing you'll notice is a drop in performance.

Use the lowest octane that you can before the engine "pings" or "knocks". That is the rule of thumb. My 1998 Maxima "knocks" when I use anything lower than 90. I use 91 octane.

Yes you will get a bit lower fuel economy with lower octane, but it is still cost effective in the long run to use the lower octane.

Let's get this straight. Your going to take the word of the grease monkey with the high school education over a room full of guys with a wall full of automotive engineering degrees and enough talent to work for Mercedes and who ACTUALLY DESIGNED YOUR CAR.

Good luck with that.

If it were me, I would stick with what the people who designed and built the car recommend. Think how expensive it will be if your grease monkey is wrong.

well since you have a supercharger that boost the compression the higher octane will burn slower to provide more horsepower.. if you start to hear preigintion or detonation go back to the higher octane fuel

shouldn't really do any harm since fuel in our days is very clean since they are mixed with chemicals that make it soo clean