Took mine apart to test it while servicing the 90. There's an oil cooler bypass which gets closed off by the thermostat when the oil gets hot- so directing all the oil through the cooler rather than just some of it. This much is evident from dismantling it.

Anyone know what temperature the thermostat is supposed to close at? Dunked mine in a pan of water and it only moved when the oil was boiling. Took it out and pushed against the pin and it contracted without much resistance, so I suspect it's kaputt. Got a new one coming anyway

I think if the oil is boiling its not working properly.