I've been looking at this for a little while. I use Torque to display three temperatures: VVT Oil Temperature (from the NADV add-on for Nissans), coolant temperature, intake manifold temperature.
So oil temperature is the truth (why its part of the three-instrument cluster I suppose), but the engine management works off of the coolant temperature.
The thermostat and engine fans are set by the coolant temperature: the thermostat moves to circulate/cool at 185degrees F. the cooling fans kick on at 212degrees F. I've watched coolant temperature walk down from 212 to 201 and then slowly crawl back up to 212 while in traffic (hey, this *is* ATL).
From what I can tell, the heat exchange capacity from the motor oil to the coolant is simply limited. At low speeds the oil temperature just seems to stick. At highway speeds, coolant drops to 195degrees F or as low as 185degrees F and then the oil temperature will start dropping.
I'll second what Raeshlavik said: I believe the best bet is the 34-row oil cooler.
I believe that there is some gain in other thermostats. I believe the Mishimoto opens at 165degrees F. I'm not certain, but I believe that some thermostat settings are programmable in the ECU. With the heat exchange limited, 5-10 degrees off coolant temperature isn't going to make a huge difference.
Good luck
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