 Oil in them. They got additives in them that they say will help stop up leaks in the system. And basically what that will do is ruin your system. The new systems take between a pound and a pound and a half of refrigerant at the most. A lot of systems only take 10, 12, 14 ounces of refrigerant. So if you mix it or you put the wrong stuff in there, you start blowing up the system.