On your LBZ CLassic you can actually run 15k oil changes with Amsoil. More often if you like, but the oil is actually good long beyond that with analysis and bypass filtration. That is a little more work than most guys want to mess with, but it is an option.
Amsoil rates the Diesel Oils (excluding the DEO 5W40) at 25k miles regular service, 15k severe service. Almost everyone qualifies as severe service, just by the nature of our trucks.
As for mileage claims using additives, its all relative. I have had customers tell me 2+ MPG, and had some tell me none. No one has told me of a drop in mileage. Mileage is so hard to pin down due to the variables.
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