l had gotten a mobil 1 oil change for $50 & my gas mileage was great then after the 3000 miles l got a $20 walmart supertec oil change & the mileage went really bad. 1 would get about 250 miles a tank on mobil 1 & on walmart it dropped to 180. Why did it make such a huge difference???
BTW l drive a 2002 honda civic sedan with 76,000 miles & l use the 5w/20 oil.

Mobil 1 has very good lubricating ability so you have very low engine friction and wear. So less HP uses just to run the motor so car uses less gas. Walmart oil is not so good.

Ive read that its not smart to go from Fully Synthetic to conventional oil.
Your best bet was to go 1/2 synthetic at least. but like the first reader said fully synthetic does help in reducing friction and heat helping your motor run better. Good Luck.
next oil change l would go 1/2 synthetic and then fully synthetic after
that.
Save some money do your own oil change. A fully synthetic oil change done by you will run under $30.

The fact that you let some unqualified guy do an oil change at Walmart is your issue here.

Because of the old saying ''you get what you pay for I'd definitely go back to using the Mobil 1. The oil is formulated to better reduce the friction within the engine, which in turn helps your engine run more efficiently, which is also why it is more costly.