[RedDevil]

Like I promised I tested the air in my tires.

When I bought one and a half year ago I raised the pressure to about 42 PSI. Kept checking and occasionally adding a bit to compensate for lost pressure.

Today I took a small plastic bag, a long nozzled gas lighter, a tiny stick and a rubber band and went testing.

First test, filled the bag with air from inside the house. Put the lighter in, ignited it, burnt fine for 7 seconds. Cut the flame when the bag began to wrinkle.

Second test, filled the bag with air from the tire (tripped the valve with the stick inside the bag). The lighter would not light. I coud see it spark, but it refused to flame even once. There was not enough oxygen to cause any kind of reaction.
Smelled the air from the bag as I let it out, smelled like rubber and unburnt lighter gas as expected, nothing else.

Third test, lighter in open air. Fired up fine, as expected.

Fourth test, held my breath for some time and then blew up the bag. Put the lighter in. Fired up just like in the other pure air tests, even though there must have been less oxygen in the bag.

So, I have only put air into the tires, quite some over time.
I am not the first owner though, they may have been filled with nitrogen initially, but I doubt it. The first year the tire pressure did drop off gradually in all four tires; this effect diminished over time to almost none now.

What is certain is that there is not enough oxygen in the tires to make a flame burn, while that flame does burn quite normally in slightly oxygen deprived (exhaled) air.
So the air in the tires has lost a substantial amount of oxygen.

The fact that the rate of pressure decline went down noticeably over time also confirms that nitrogen does in fact seep out less easily.
I did not expect that.
Myth confirmed, I guess.

So, apparently there is a benefit from filling tires with "nitrogen enriched" (oxygen depleted) air. They will keep their pressure better than tires filled with plain air, as those will lose most of their oxygen.
Having said that, you will need to keep an eye on the tire pressure regularly regardless of what they are filled up with, and in the end you will end up with almost pure nitrogen anyway.
You will just need slightly more air than if you use pure nitrogen.

As for better ride quality etc; obviously even if it does make a difference it is hardly an issue as the oxygen will get lost anyway.