Use the thread type and weight expected by the embroidery design, then check the machine and needle guidance. General-purpose sewing thread and embroidery thread are not interchangeable merely because both come on spools.
A documented starting point
Sulky’s thread-and-needle guide explains the roles of its thread types. Sulky 40 wt rayon embroidery thread is one identifiable option for designs intended for that weight. It is not the same product as an all-purpose sewing polyester or a heavier decorative thread.
Search Amazon for Sulky 40 wt rayon embroidery thread
For another fibre, compare an explicitly labelled 40 wt polyester machine-embroidery thread whose maker supplies needle, tension and care guidance. Check the exact product rather than assuming a brand’s general sewing thread is its embroidery line.
Dense designs and satin stitch
A design digitized for a particular thread size assumes a certain coverage and stitch density. Switching to substantially heavier thread can crowd small lettering and narrow satin columns. Switching finer can leave gaps. Test the complete combination of design, fabric, stabilizer, needle and thread before embroidering the final piece.
Do not interpret labels such as “12 wt equivalent to 50 wt” as interchangeable specifications. Thread numbering systems differ, and a decorative multi-strand yarn is not automatically suitable for a home embroidery machine.
Durability and laundering
Compare the thread maker’s care requirements with the finished garment’s intended washing. Do not promise that every polyester thread is bleach-safe or that every rayon thread will fail in washing. Use documented instructions for the exact thread and colour.
Buy a few colours for a test design before ordering a very large assortment. Record the colour code and lot where available so reordering does not depend on a marketplace photograph. Check spool length as well as price per spool.
Use the stabilizer guide for backing and the beginner embroidery-machine comparison if you are still selecting the machine. A thread change cannot compensate for an unsuitable design or poor stabilization.