Here's a thread:
http://www.artglassanswers.com/forum...php?f=21&t=167
Most of the thread is in agreement that propylene can be used, but the last post said that if propylene is used, the manufacturer will no longer honor the warranty.
There is also a reply on these forums that propylene is acceptable, but I can't find it. In that thread, the warranty issue was not addressed.