On my 01 E320 I have always run 91 and I track every tank of fuel. Several months ago I decided to try running 87 in it. I ran it on 87 for probably 2-3 months, then ran it for about a month or 2 on 91 again. I can tell no different in power and MPG is the same. I am getting anywhere from 25-28mpg with both types of fuel. I have talked to some folks who run these only on premium and others say it is perfectly fine to run them on 87 but you might have lower power and less mileage. But, as I tracked for myself, power seems the same and mileage is identical. Anybody else tracked this and had similar experiences?