Now if you change it to something else and the gas mileage does not go up then the Havoline permanently ruined your engine.
Highly doubt its the oil but this is not the first story I've heard like this with <insert your budget oil here> causing a sudden and immediate gas mileage drop.
What oil did you use before? Are you using the same gas station, same driving pattern, same lead foot. One good goose off the red light can drink quite a bit of gas.
I would benchmark it really well at the same pump (miles / gallon / tank) over several fillups. Then switch back to your favorite oil. Test it for a few tanks and measure at the SAME pump.
If that increases your gas mileage back to where it was before, then you have good reason to not use havoline.