Again referring to the service manual for the XC I can see no thermostat in the oil system. Yes there are 2 pressure relief valves, one for the lubrication circuit and one for the cooling circuit, but no indication of any temperature sensing and subsequent flow control. As well there is no mention of temperature sensing capabilities of the pressure relief valves.
With cold oil requiring more pressure to move, it is possible for the relief valve to open thus bypassing the oil cooler. I guess in a way this is indirectly temperature controlled flow.
Of course this could be entirely different on an 06 Vegas.