To the best of my knowledge, they all use hydraulic assist P/S.
I'm new to the air game, but I've never heard of air assist P/S, and considering the amount of lubrication required in the P/S box , I can't see air assist being possible; unless the assist was external to the box( air rams on the steering linkage ) then control of the direction and amout of assist would be compex . To say the least.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.