propane has a slight amount of oil in it. it will collect moisture. that is in the system.
i installed a low point drain in my system where the flex hose connects to the black piping. once a year i turn off the tanks. remove the cap from the tee fitting i installed and let the oil and water milk drain from the system. clean and re-install the cap. open the tanks and check for a leak at the cap with soapy water. i also check the water column pressure of the gas, adjust the regulator as needed. my system very seldom freezes up in cold weather and gas flow is even.