I guess air would work in the bladder, it simply wont have the temperature stability that pure nitrogen has, because of all the moisture in it. It might cause a corrosion problem, though.
If the shock has been used with no oil or nitrogen pressure in it, I doubt it is going to hold any pressure anyway, the bladder is probably destroyed, along with all the shock internals.
I use ATF type-F in my fork.
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1992 KDX 250-FMF porting,two-stage power reeds, Fatty pipe, Power Core silencer,titanium rod,Wiseco Ultra-lite, Pro-Action suspension...Oh my Gawd, they killed Kenny!