Oil in propane line

I had it happen years ago while running a portable Coleman heater that was designed to run on 1 lb bottles, but I was running it on a genuine propane hose from a 16 gal(?) cylinder, and I was running it continuously. I don't remember it giving any trouble while burning, but when I changed the first bottle, I remember the mess of pitch black oil that came dripping out of the hose, and the heater. I remember cleaning it all out,.....only to have it happen all over again on the next tank. At that time, it was said that the oil comes out of the rubber hose, which I did not believe a word of. It would have been impossible for that much oil to come out of that hose,....never happened ! I believe it was in the propane.
But, I have NEVER had it happen again either. Ironically, I've never used "propane hose" (whatever that mite mean) since then either.