Dave's knowledge trumps my basic understanding of things. The compatibility between "varnish" (A non-oil based product) and poly is new to me. I'm looking forward to learning more about this phenomenon............
Poly is a varnish. A urethane resin varnish. The most common other resins used are phenolic and alkyd. Waterlox is an example of a phenolic resin varnish.
Just to be clear, when I mention an varnish-oil blend, I mean that the product consists of a varnish (a chemical combining of resin and oil) mixed with oil. More often than not, this is done for appearance reasons. These dry softer than a varnish alone. Of course how hard a varnish dries varies from one product to another.
If you haven't read it, check your library for Bob Flexner's "Understanding Wood Finishes." Or buy from Amazon. It's worth it.
Here's an informative article by Flexner about so-called oil finishes.