There isn't a thermostat in the 5HP - it relies on the engine temperature. The 'oil cooler' in the radiator is really properly called a heat exchanger. It uses engine heat to warm up the transmission to 90*, and then dissipates heat generated by the gearbox in to the coolant once it's up to temperature.
The oil definitely does expand with temperature. The way I fill them is to top up to level when the gearbox is cold and then run it until 40*. It will drip oil while it is warming up as the oil expands, and then as soon as it hits 40 I put the plug in. Much easier to do it that way rather than wait until its at 40 then put fluid in, since its really easy to over-shoot the temperature like that.
As a side note, the 3.7 with the 5HP19 has an external thermostatic oil cooler in addition to the heat exchanger in the radiator, because the 5HP19 with a V8 on it is right at its operational limit so needs the extra cooling capacity. The 2.7TT and V8 Allroads have the same arrangement.
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