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I run PowerService in every tank (white bottle in winter, gray in summer). gray bottle seems to help MPG by 1-1.5 in the summer and white seems to quiet the injectors a tad in the cold. Any type is cheap insurance. Members with more smarts than I will chime in and give you specifics of why you should or should not.
I've been using PowerService DieselKleen (Gray Bottle). I don't have enough MPG Data yet to tell you one way or the other, but my idle is quieter now.
I didn't realize about fuel additives until recently. Apparently, a lot of troubles with modern idesels come from the fact that they're designed for higher-quality fuels than the older designs were. I read a lot about carboning up, and folks seem happy with the additives.
I'm going to do a couple of cross-country runs again this summer, and as always, I'll try and get my MPG data up on here at that time.
The two big names bandied about up here on FTE are PowerService and Stanadyne. Both seem to work out well.