If you want to continue reaping nitrogen benefit, never ever pump in normal air. Coz the moment even a bit of normal air goes in, chain reaction...the nitrogen essentially becomes normal air.

If lazy to always goes back to shop to pump, just convert it to normal air but pumping at gas station lor. For my City, I used nitrogen and overpump it to 3-4 PSI higher than usual. Last a couple of months before next pump. For my PE, I don't bother, normal air cukup, easier actually although a good nitrogen air does makes the drive slightly ligher especially on the steering and most importantly lose air slower.

ps. but then again, all these feeling business might as well be psychology feel, no prove, maybe not even using real nitrogen air since lots of these shop never bother to maintain the machine and nitrogen pump in already contaminated with normal air.

Like this soha1? 

lol.. Of cos feel better la. Nearer to nirvana ma