I bought a wonderful Singer, circa 1950 ish,
at a wonderful price and she will only let me use Sylko, which is the older Coats n Clark's cottons. These were discontinued years ago but fortunately I can still get hold of them via EBay etc; and there is nothing wrong with the thread at all.
IMO, and many other quilters, it was the finest sewing thread ever made, so much so that Coats have bought it back as "Coats cotton" it's expensive and IMO nowhere near as good as the original. So hang on to them and use them on special quilts.