I have seen small amts of "oil" with my trailer.
No real problems though. I always thought it was from the hoses.
This was reinforced when I saw that the Mr Buddy heaters sell a filter if using larger than 1# tanks, or a green hose that doesn't contaminate the regulator.
This seems to happen with high pressure hoses with the regulator at the appliance (heater, grill).
95 2500 4x4 Dodge CTD
96 Alfa See Ya 30RK