I have a set of variable contrast filters I bought on eBay a few years ago, when I first got back into photography and darkroom work. They're pretty old, made by DuPont for use with Varilour and Varigam, two papers that haven't seen the light of day in almost forty years. They seem to work okay, and I never gave any more thought to it.

But then I read this line in a Freestyle catalog, describing Oriental Warmtone VC FB: "Compatible with Ilford Multigrade Filters."

What does that mean? Why would I need Ilford's filters to use their papers? What "incompatibilities" might my DuPont filters have with VC papers made by Ilford and others?