[42pilot]

Cetane has nothing to do with economy - just like octane in gas has nothing to do with economy. Cetane, like octane, is the fuel's ability to burn predictably and controlled, rather than explode. For your info, the US has one of the lowest cetane ratings in the world and continental Europe has some of the highest (40's vs 50's respectively). Higher cetane really helps with lower temps and the first start of the day.

Our biggest problem is contamination - water - in the fuel. The direct injectors used in these engines require: 1. lubricity, until a couple of years ago was solved using high sulfur content, and 2. highly filtered fuel. Current fuels replaced sulfur with other lubricating chemicals that newer injectors can use well. Older motors with older technology injectors have a little more problem using the newer fuel and seem to fail sooner (usually around 75,000 miles which is fairly low for a diesel). Today's injectors should last 150,000 easy. Water will kill an injector because it has no lubricity and you'll start to see the pintle of the injector stick.

As for where to buy, I always buy a name brand and where there is high volume in terms of sales. You do not want to buy fuel that sits for weeks in a tank due moisture accumulation.

You can buy additives that really work for Diesels, but I am not sure if they are approved for use in BMW's motors. Anyway, the good additives boost cetane and add lubricity to the fuel. For my Ford Powerstroke diesel, I add 2 cycle oil (1 qt per 22 gal) for lubricity. At +$250 per injector, this is cheap insurance for me.

DEF is not water and the price of DEF is not worth the problems the substitution could bring. Don't even consider it, especially in a +$60,000 vehicle. If you can't afford DEF refill every 10,000 miles or so, you have bigger problems.

I wouldn't stress over fuel like gasser people do. Buy from a name brand and make sure you see lots of people buying diesel there. FYI - I buy mine from the local Kroger because they buy theirs from the same place that sells to Valero, Marathon and Texaco, and you always have to wait in line, regardless of the time of day.

PS - I am averaging 28-ish in comfort around town and 31-ish on the highway. I don't use Eco-whatever...