A brick wall maybe compared to a full flow, but I removed the restrictor orifice so the flow may be more than we think. The way I see it, the only restriction is the density of the toilet paper.
I don't think you have too much differential across your heater core. The 4.0 (and I assume the 2.5) bypasses a decent amount of flow. That's how they get the things to warm up so quickly for a ancient iron pig. My wife's 4.0 is way faster than my aluminum head 3.0 Mitsubishi. It's almost like the radiator "also ran" in the design of the system. The water pumps must be of pretty high output to manage this.
My 0w-10 experiment is using the coolant flow to heat and cool ..but there's no filter involved. I need to do some rerouting. An update is scheduled ...soon.