the price difference really is only 2 bucks but if you are that picky then yes i believe that 89 is ok to run on... your fuel cap area should read a minimum to be certin but i wouldnt go below that... your engine will adapt to the lower octane but it can only retard so much...
I use 91 here in Canada. I'm not sure how gas are rated in Europe but the thumb of rule which I learned while driving my bimmer is to use premium quality of gas. I tried using 89 and car was crying for higher quality.
In Europe the grading is normally done on RON, as opposed to MON or PON, which, I guess, is probably what they use in the Americas.
For a 318i, I think 95 RON (87 MON / 91 PON) should be ok. But there is no doubt that a higher graded fuel will be more beneficial. Increased power, efficiency, and some have additives that will clean the fuel line and injectors.
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