good question barnbum, I'm thinking the same thing myself....and I believe that the all purpose coats & clark is poly/cotton blend....I'm not in the sewing room, so feel free to correct me if I'm wrong.
I learned on an old Kenmore and was told it was fine to use all purpose thread. I purchased some very nice polys for the embroidery machine and was given zero advice about the recent Kenmore DH purchased for me as a surprise. I have Gutterman because mom likes it best but on on these projects I'm giving away, I'd like to save a bit of money on thread so I can spend it on fabric/batting/tools.
I've had my current Kenmore for a couple years without maintenance (I know, I know) and haven't had any issues. The embroidery machine (Janome) is riddled with problems and constantly clogged up. I put in the closet because I just got so frustrated....I wonder if I've been using the correct thread all along. ???