[Secret Agent Man]

For most people it makes zero difference. The air has about 80% nitrogen in it by default.

[Scrambler Bob]

The air has about 80% nitrogen in it by default.
= = =

Oxygen molecules are smaller than nitrogen molecules.

If they ‘leak’ out of the tire more than nitrogen, the nitrogen % will raise.

My experience is that after filling a tire, pressure decreases over a couple of months (and is topped up), then stabalizes.