Pure nitrogen leaks out slower than the O2/N2 mix that is air- oxygen is slightly more soluble in rubber than nitrogen. Maybe sulphur hexaflouride? It's a big dense non-reactive gas that may have trouble going through rubber. Might be hard to obtain, tho. Also, "heavy-duty" inner tubes are less permeable than lighter ones strictly from their thickness. But no, any gas you put in your tire will come out eventually.