First up dave, Kinematic viscosity is a function of density, so as the density changes this also affects the kinematic viscosity. Dynamic viscosity changes but doesn't drop off as fast.
Secondly, hiro, You're not talking about a fixed volume. Gear pumps are positive displacement but they are a clearance fit only. As your oil gets hotter, the effective sealing of the gear pump is reduced as the viscosity changes and it's not pumping as effectively, so that's partially why your oil pressure will be lower as it's hotter.
The point I was making is that you want to pass to the oil cooler as the oil heats up. Counterintuitively, using a pressure circuit will be less likely to bypass to the cooler as the oil gets hotter.
Porsches run a secondary cooler loop and they're thermostatically controlled, not pressure control. And seriously, as the oil temp rises the oil pressure gauges in mine get frighteningly low. 911's run 2 thermostats as well, one for the primrary cooler and one on the secondary cooler.
EDIT - that isn't to say that there aren't pressure circuits set up this way.... Just that it'd much much smarter to run it off Temp!
EDIT EDIT not that it matters we're getting off topic sorry Cam.