I saw this in an article about clothing detergents in yesterday's New York Times, "Cold-Water Detergents Get a Cold Shoulder":
The chemical composition of the new cold-water detergents, which cost about the same as regular detergents, is “totally different” from what was found in detergents a decade ago, said Dr. Mueller-Kirschbaum of Henkel. Some even contain chemicals that coat fabric fibers so that they are less likely to absorb dirt in the interval before the next washing.
Those would be bad detergents to use in washing any sort of fiber before dyeing. It has not really mattered too much, in the past, whether one used Synthrapol
or a more ordinary laundry detergent, but any detergent that coats fibers so that they absorb less dirt will be a very bad thing for prewashing before dyeing. Maybe not such a bad thing for regular laundry after all dyeing is completed, though.