Oil residues plugging propane lines has been a topic of discussion for years. Even the big boy propane companies can't agree on exactly where they come from. Here's a link to
one discussion on the subject if anyone's interested.
Most of that discussion has to do with other uses than in RV's, but there's one point made that's worth paying attention to.
IMPCO wrote:
During the process of vaporization some heavy ends will collect within the regulator over time. This can be reduced by mounting the regulator with the outlet pointed down
That's evidently one reason the regulators are meant to be installed pointing
down, and checking for proper positioning is well worth doing.