You can buy a water separator from AmsOil at their website, but unless you drive cross-country regularly, encountering varying fuel qualities, and degrees of humidity, they really don't make much sense. And, you probably don't need one. Driving in the same relative area, and using a quality diesel fuel while keeping your tank refilled regularly is what really matters in these terms...

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Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."