While propane may have some residual oil, I believe most comes from hose Plasticizer Contaminate. Warm weather causes high pressure in propane containers, the pigtail attached to the container to the red 30 psi regulator experiences this high pressure, 90F = ~150psi, 80F ~130psi, 60F ~90psi, this high pressure causes the plasticizer in the hose to leach into the hose.
https://www.propanecouncil.org/uploadedFiles/Council/Research_and_Development/FS_11297%20Propane%20Fuel%20Quality%20Improvements%20through%20Hose%20Conditioning.pdfGig em!