I have not tried Petron Plus, but there has been a lot of debate on whether or not oil additives work. Many tests have shown a small to moderate benefit from oil additives, so I imagine using them wouldn't be so bad as long as the cost/benefit makes sense. I don't know specifically about Petron Plus, but testing the benefits of oil additives is very costly, so I would stick with large companies that can afford to test (Lucas Oil, STP, Slick50, etc.) That being said, anything you can do to make your engine more efficient will result in less gas used. SCT has a computer tuning devices that can re-calibrate your engine to be more efficient. Diablosport also has a tuning device that actually allows you to run E-85 ethanol in some GM vehicles. There are also high-flow air filters, like those from K&N, that can help an engine breathe better.

I have noticed that by using higher octane fuels and especially Shell gasoline I get better mileage out of all 3 vehicles. I assume that it burns the fuel more completely, giving me better performance, and also leaving less unburnt gas to escape through the exhaust system.