N2 expands less that air with increased altitude?????
That's definitely a myth - both 'air' (~79% N2/21% O2) and pure nitrogen conform nicely to the ideal gas law - which basically says all gases react the same to changes in temperature and pressure.
Someone once told me that N2 molecules are bigger than the gaps between the rubber molecules therefore they hold the pressure better. Not sure if that is just an urban myth.
My gut feeling is that it's a myth (the molecular weight of N2 is lower than O2, so everything else being equal the N2 molecule would be slightly smaller than O2) but there could be some subtle way the molecule reacts with the rubber that makes it effectively larger. At any rate the difference would be in the mud