I am not an expert, but have used the sulky variegated thread. When I was buying it, I did note that some was 30wt and some was 40wt. I just looked and I have Sulky that is 30wt, and YLI that is 40wt and Signature that is also 30wt. I have not had any problem with any of it, in fact I recently quilted a QFK's quilt using the Sulky 30wt and had no problem. I do think that I used a slightly larger eyed needle. I didn't use a fancy stitch with that quilt, but have used the same thread with a fancy stitch and a larger need with no problem. I also went a little slower than normal.
Here are some links to info on needle size and threads:
http://quiltbus.com/machine-needle.htm
http://www.quiltbug.com/articles/thread-theory.htm
http://www.sulky.com/faqs/faq_001.php
Hope this helps.