I have always used 40wt - I just ordered some 35wt to try. It is heavier than the 40wt, and is used both in bobbin and upper threads. Good sewing.
40 weight Rayon thread will give you a softer lace.
40 weightPolyester thread will give you a stiffer lace.
A good 50 weight Cotton thread will give you an old fashioned or Heirloom look to your lace.
I use 40 Weight Rayon for most of the lace designs I stitch and some are very stitch intensive.
EastWitch2
Almost all of my FSL projects have been made with RA and I haven't had any problems with it being too thick.
It depends on a lot of variables. What effect you want-shiny, cotton like hand look. The digitizer- some make designs that will be stiff no matter what, and some end up flimsy after washing out the wss. What the design is for- Ornaments usually you want to stay stiffer, lace you might want softer. You can try soaking with some fabric softener if you want a less stiff project.
I use either my poly or rayon, which are both 40wt. I have been trying some hexagons with some serger thread to try to make a tablecloth, but haven't been on the project for a while. They were a different texture than the rayon.
You'll just have to try different combinations of design/thread to see what you like best.
Jo
Wait, some friend already is going to help you. I do not know. I am very new in the embroideries.