most propane has a fine oil in the gas… it usually settles to the bottom of the tank. but during movement or refilling it remixes…. a good preventive. is to purge the last little bit of gas from the tank after moving it around to get the oil mixed in…
another method is to install a low point drain in the supply line… on my coach, i placed a 't' fitting where the flex hose connected to the black metal gas line… using the extra fitting to use for the barbecue as needed. otherwise it was the low point drain… as oil would collect in the vertical line under the fitting. between the fitting and the cap on the fitting it would hold about a teaspoon of oil. if the problem was severe you could connect a short capped hose or pipe to hold more oil...