I have been using Howes for the past year, around here it is found at truck stops and costs about 9 bucks a quart. I have ran without it and noticed a difference in sound and mpg. Just my 2 cents.

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Habits I support
95 E300 D 220,000 & counting (I think I'll call him Fritz)
02 Tahoe (Momma's ride)
98 GMC Sierra 120,000
57 Chevy Nomad (bought in '69)