Fuels with additives that have passed independent testing are listed here www.toptiergas.com/retailers. I do not see WalMart on that list. The most tested and accepted fuel additive among experienced technicians is Chevrons' Techron product. Fuel additives can have an affect fuel mileage and engine life; how much will be the subject of opinion as there are many variables.
Comments on Additech, the additive that Walmarts website lists.
One assumes a company the size of Walmart would have thoroughly tested the product. The claims made are significant but, except for the mileage improvement, would be challenging to prove.
Claims and comment:
1) Cleaning combustion chamber, intake valves and fuel injectors thus reducing engine knock tendencies: This is an excellent goal. It could improve mileage if the engine has enough deposits to cause the engine controller to have to correct for knock. Top Tier approval would ensure this claim.
2) Friction reduction modifier: The claim here is that the product contains an ingredient that reduces piston ring friction and gets into the engine oil and then reduces friction throughout the engine. Proof is required for this. Others have made this claim and been proven wrong over the years. Modern engine oils are excellent stand alone products not in need of additives. Also, most if not all, OEM's do not approve oil additives and friction modifiers as they can damage the catalyst over time.
3) Anti-ice additives: This is usually alcohol.
Should you buy it? No, due to the lack of certification and the unapproved oil additive friction modifiers.