I have a Janome 350e embroidery machine (have had it for 3+ years). The dealer sells all brands of thread, but she highly recommended that I use Madeira Polyneon. She said rayon is not as colorfast, and does not hold up as well when ironed with a hot iron. I have never had any problems with the Polyneon. I buy it from thesewphisticatedstitcher.com - the prices are the best ($2.65) and the service is amazing. If you buy 48 spools of thread, you get a free thread case.
Here's the link to the conversion chart I use:
http://threadchart.info/threadchart.php?f=MADPOLY7