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I think you have very good handle on things. I've been through the air filter and change to synthetics on a gas 4wd Silverado and I didn't see any improvement in mileage. Synthetics may show some improvement in winter. That was a good point. I did, however, see a substatial improvement after putting a three inch straight through muffler and headers, surprisingly.
I have noted a small improvement on my 06 TDI with fuel additives. There can be a huge swing in mileage related to fuel quality though. I usually hang at 40mpg in summer and 38 in winter, but I got 32mpg once on what I'm sure was a poor quality fillup once. I got 46mpg 90% highway this year since I found a very good diesel supply. It gets tested often for contaminants. Much of that jump was because I'm in mixed driving mostly.
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