I'm not sure why exactly it would be bad in an air compressor oil.. but here's some nuggets of information - car engine oil has used phosphorus and zinc (zinc dialkyldithiophosphate or zddp for short) basically as a dry lubricant that gets left behind on high pressure metal on metal surfaces when the oil film gets squeezed out. The ppm of zddp in engine oil has been drastically dropped in recent years because of pollution issues where burning oil will poison the cat with zinc/phosphorus deposits, but it's main reason for being in engine oil was from the old days when they had flat tappet lifters and no cats to worry about. Now everything uses roller followers, so it's not as much of an issue, except when you build hotrods with pushrod engines that have flat tappet cams. The reduction in zddp in the oils will make it flatten out cam lobes really quickly unless you use a zddp additive or oil specifically formulated for that type of setup