[RedDevil]

About the theory that the oxygen inside a tire will get out faster than the nitrogen:
That is very easy to test.

All you need to do is release some of the air from one of your tires that you've pumped ages ago and collect it in a large plastic bag.
Now light a lighter inside the bag, releasing as little as possible.
If indeed the oxygen has escaped the lighter shoud not burn at all. Not if there is less than 15% oxygen.

It will ultimately run out of oxygen if the bag contains pure air too, of course, but not immediately. I predict it will do just that with any air from an air filled tire, and even with some so-called nitrogen fills. Unless every nitrogen pump holder is honest and not interested in a quick win.

I'm gonna do this test on my tires tomorrow if weather permits. My summer tires hardly leak at all, most of what is in there has been there for over 2 years now.

EDIT be pretty damn sure there are no combustibles in the tire before you do something like this! Ever used a tire repair kit on a tire, then keep away fire.
Test in the bag, not at the valve. An explosion in a thin plastic bag is just a bang. An explosion confined in a tire may cause serious damage when that rips.