I wouldn't put air in a shock. Nitrogen is used as it is affected much less by temperature (changes the pressure) and is dry so won't affect seals and internal parts.

Yes, you are correct. Nitrogen is better. But how much?? Let me see; ambient air is at least 75% Nitrogen in the first place. The thermal expansion differential between "pure" (actually about 95%) Nitrogen and dry air is so small it is virtually undetectable. As far as corrosion goes, yes, that is a problem. How much so? Well, I worked on aircraft hydraulic systems for the Air Force for almost 10 years. They used dry air for the last 50 years, and switched to Nitrogen a few years ago because some of their 30-year old aircraft were showing signs of internal corrosion on the landing gear struts (the landing gear struts use air on top the oil as the internal "spring" ranging from 1000-3500psi).

I dont know about you, but I service my suspension about once a year. Never had any problem in streetbikes or dirtbikes using "dry air" instead of nitrogen, and save a ton of money by it. Actually, I would bet good money that my suspension will last longer than most because I change the oil more often.

Price-wise, I can service my own shock for about $3, not counting the initial $20 for the pump.
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Paul Herr
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