[Gustav]

In motorsport,the reason for using N2 instead of air in tyres is due to the H2O vapour making your pressures more inconsistent as the temps increase/decrease rather than the O2 or CO2 content.

Not so critical for a trip to the shops though.

They wouldn't worry about oxygen content making rubber brittle in a tyre with an expected life of less than an hour so that can't explain it's use in racing.(anyway, what about the air on the outside?)

It's just because of water vapour messing up your pressures i always thought