They posted a report on Frybrid's forums about how veggie oil polymerizes in a bunch of different environments. One of the conclusions is that copper in contact with VO leads more quickly to polymerization than SS, with aluminum being the least reactive. Heat is also a factor. I dunno, haven't done any experiments myself, just re-stating what's in that report. If you're having good luck, keep doing whatever you're doing.
Here's the thread with the report, and you can read all the different arguments people have interpreting it:
http://www.frybrid.com/forum/showthread.php?t=6510