My Range Rover came with the green caps and Nitro filled tires. I had to get one topped off and the tire place did it for free. I asked him about what to do if it gets low again and there is no nitro around he said just put air in it. Not going to make a huge difference but will allow the pressure to fluctuate a little more.

My Rover has nitro and the wifes has air and with the temp changes we have had this winter her sensors have been going off every other week or so and mine have not gone off once since I had the tire topped off. More of a convenience thing if you ask me.

If they ever start charging for it though I am going to air.