If your gas has any ethanol content (near me, its 10% ethanol) you will want to be a bit cautious. Draining a tank can be a bit laborious. I would recommend a couple cans of "heet" or "dry gas" to absorb any moisture (and ethanol attracts water) to dry it up. I would drive a bit carefully untill the tank is used up., perhaps mixing with "fesh" premium along the way. Could be you will notice nothing.
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