I'll add some new info....I went cheapo and tried midgrade in my 300E, and well well well, it runs perfectly fine/the same on it. I got 22mpg going 74mph for 250 miles with the A/C on full blast and the car full of camping equipment last week. I won't use regular, but since Midgrade works, that's what I'll be using....engine runs/sounds/feels/performs exactly the same as on premium....in fact, I think it idles a bit better and starts a little easier. I still use the redline in it to keep it clean though.

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'13 ML350 Bluetec - 95k - dad's (OC-98k)
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'16 E400 4matic Sedan - 148k - Brothers (OC-155k)