While there are Teflon lubricants, Teflon (PTFE) thread seal tape is most definitely for sealing threads. There are relevant standards for these and they are UL listed for such use. Typically for propane & natural gas you would want to use yellow thread seal tape. It is thicker than the standard white. I prefer it over "pipe dope" as it does not harden up with time, and it is guaranteed to have even coverage. In addition to it's ability to seal, the lubricating effects of the tape allow easier tightening of joints, further reducing the liklihood of a leak.