External oil thermostat - how does it work ?
 

Mostly out of curiosity, how does the ext oil thermostat on a 74' 911 works ?

Pressure ? Viscosity ? Bimetal sensor ?

It responds to oil temperature. There's a special material spring in the valve whose force changes with temperature.

As temperature increases, more oil is directed to the cooler.

In valve-speak, it can be called a modulating 3-way valve.