I have a 1994 GMC c1500 with a 5.7 liter 350 vortec and it seems to get terrible gas mileage, like 9.5 on a good day. I’ve put two cans in the full 26 gallon fuel tank. Should I do this again? Also I have put Sea Foam Spray down the throttle body but it is separated into two barrels. Do I use a can in each, and do I have to change the spark plugs after?