If it was debris, it should be in the media, not plated-out on the bottom of the canister, bottom of the cartridge and all over the bypass valve assembly though. That's my concern. Also, have no way to easily check what things look like elsewhere in the engine unfortunately.I'd say it's debris that the filter caught. Doubt it's actually "varnish" ... if it was, then there should also be the same varnish build-up going on inside your engine.