Bacon, are you confused enough? An oil free compressor should be fine for OC air. About the humidity thing, we are hearing that ANDI requires -50F? Jim didn't say, really, he just said -50. Grade E requires -60F (24 ppm) and that is extremely dry. You don't need "modified" grade E because Grade E is good enough in practice if not in theory. Fact is that most compressors with modern filtration systems produce air which exceeds Grade E criteria. In the case of the oil free we are more concerned with moisture and I will discuss that. The modifed E recommendation for OC air is a product of NASA and sellers of filtration systems which is passed along to students by fellow travellers called "instruction or standards agencies". Regardless, users of small, oil free compressors often overlook the humidity requirement and also the need for enhanced filtration required with gasoline engine driven machines. So, there is a presumption that users of oil free compressors are a little naive or careless about the definition of good air. This behaviour is primarily due to propaganda put out by RIX, a manufacturer of these compressors who brags that their machines emit no oil and suggest that wet air is good which is a fundamental misunderstanding of diver requirements. (They charge extra for dry air filtration). However, a dive shop would be the subject of numerous complaints if they were at fault with respect to moisture content of air. Bottom line, go ahead and top off your OC tank at the shop.