Not sure about the better gas mileage claim, but you can expect better tire life if you use Nitrogen according to most sources. This is a little scientific for me but, "Experts in the tire industry indicate that oxidative aging is one of the primary causes of decreased tire life. Oxidative aging is caused by the diffusion of oxygen from the pressurized air cavity of the tire to the outside atmosphere. Tests have shown that if tires are inflated with nitrogen, there is a significant reduction in tire failure."

Adding air from the gas station won't be a problem.

Nitrogen doesn't react to heat or cold like air does. So the tires won't expand. Nitrogen will maintain a consistent tire pressure much longer. Its the consistent tire pressure that will contribute to better gas milage.

I'm a nitrogen user for a number of years. Most all the claims are bull $hit IF YOU CHECK YOU TIRE AIR PRESSURE REGULARLY AND AT OPERATING TEMPERATURES. A couple of things I know it is mainly the WATER that gets into your tire from air compressors that cause the issue. What is the air made of? About 78% Nitrogen. Anyone have a tire fail due to an INSIDE oxidation? Not before the tire is worn out. What about the Nitrogen Molecule is bigger and air will not 'seep' out of the rubber? B.S, check your tires pressure. So do I run it? Hell yes, and here's why. I can get a tank of it and take it to the track. It is generally dry from the tank so pressure due to water vaoprizing is minimal and I can get a lot of use out of a tank vs. an air compressor type tank. You still need to check them at operating temps. The other issue is that unless the tire is completely evacuated (vaccum) 'wet' air still is insided your tire.

You can mix regular air no problem, just check the pressure!