Walmart, farm supply store, marine supply store, major part stores (you may have to order it but not pay the shipping), farm tractor dealer, RV dealer (iffy), truck stop (iffy), fuel wholesaler (ask your station who they buy from). The generic name is biocide. You want just biocide, not a jug of fix-it-all with a little biocide in it. Buy a quart and it will last a long time, especially if you do this: shock the tank, drain the tank into fuel jugs, filter that fuel so it's clean again, clean the fuel tank strainer, put fuel back into tank, prime at least a quart before changing the filters. Keep the tank full and treat the fuel (I use grey bottle PowerService). And keep a few spare filters in the trunk. A few hours of work at home is a lot cheaper than a few on the road.

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daBenz - 1970 220D