You could presume that viscosity will drop and compressibility will surely go pretty high with just a little air in it. Sorry, no hard data on it. I'd tend to assume the same compressibility for the volume of air in the mix at any time (considering pressure changes) ratioed to virtually zero compressibility for the volume of oil you had in any given volume of the total mix. Kind of treating it as a 2-phase flow, but as separate phases for each fluid. I don't think that the viscosity would be affected so much at low air ratios, but at very high air ratios it might be, depending on what type of flow you had.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
