Changing The Oil and Gas Mileage?

I have a 1988 Ford Ranger V6 2.9L. I have had this truck since about March of 2007. When I first bought this truck I could run about a week for $25 and at a gas station with gas the same price it was when I bought the tank it cost me $40 and thats if I drive the same number miles each week. I changed the oil today... show more I have a 1988 Ford Ranger V6 2.9L. I have had this truck since about March of 2007. When I first bought this truck I could run about a week for $25 and at a gas station with gas the same price it was when I bought the tank it cost me $40 and thats if I drive the same number miles each week. I changed the oil today and the old oil was solid black. Could the old oil that was in it be what was causing me to get such bad gas mileage?
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