So, some aliens discover that human skin oil has unique properties that are desirable for them. They cannot synthetically synthesize it for whatever reason.
So, to be clear, they really want the stuff. Nothing else will do, no animal oils or synthetic products. They need naturally produced human sebum, as fresh as possible.
How could the oil be effectively harvested from humans? Preferably without killing them. It doesn’t need to be comfortable but it shouldn’t kill the person involved.
The best answer would describe a method that maximises oil production. How much could they expect to get from a person each day? Of course, there’s a lot of variation here but I need ballpark numbers.