If the car says to use it, then you use it. BMW gains nothing for whatever fuel you're using it after the fact, so it's not like it's some conspiracy theory to make more money. I accidentally put 87 into my 740iL and the car fell on it's face...bucked and stumbled, felt like someone dropped an anchor when I went WOT, just didn't run right. The fact is the engine was tuned on higher octane for a reason. Just like you can tell the difference between slim milk and whole, your car can tell the difference between 87 and 91 or higher. I explained it like this to my mother...

It's a BMW, so it's higher compression than her Honda. Higher compression means it's runs hotter. Higher octane fuel burns slower, which means it helps cool the engine for that microsecond before it's burned. Can she run 87 octane, yes. Will the engine run hotter, which means it'll ping more, which means the computer pulls timing to make up for predetonation, which means she'll have less power AND lower gas mileage, yes. She can either pay .40 less per gallon and get 20MPG or she can pay .40 more per gallon and get 24MPG (theoretical numbers), but ultimately you're better off following what the manufacturer suggested because it was made for it.

If it recommended 5w30 oil but WalMart had 15w40 on sale, are you going to put in the wrong oil to save money? It'll cost you in the long run, so pay now or pay later. Choose your poison.

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