Bewy

I have only owned my 2000 model W203 for about 2 months, & have used 98 octane fuel since purchasing it. 2.6L V6 engine. It is also my first MB. I filled up with 95 octane last week & the car feels a little sluggish now. Maybe my imagination, I am not sure? NEITHER FUEL CONTAINS ETHANOL. Also the octane rating method differs here from the US & other countries, US octane is about 4 points lower, so would be 93 & 90 respectively in the US.
Is it possible that the lower octane rating has affected performance? Thanks. Anyone else noticed the same thing?

ncmudbug

I can't speak for the difference in the octane in your gas and ours in the US but there is no cost advantage for buying 89 (mid range) over 91 (premium) using the RON method of calculating the rating. The 89 gives worse gas mileage so you end up buying more gas. It seems likely to me that with lower gas mileage, that performance would suffer also, but I didn't notice the difference - but then I mostly did plane highway driving and might not notice it.

Alswag

Lower octane does affect performance as lower octane fuel does not combust as well as higher octane fuel, making your engine run less efficiently.

I always put 94 octane in my car (Canada, regular is 89, premium is 91, high-premium is 94), and I never see a dip in gas mileage. She's always running clean.