All,
As per subject. Thermostat or not for an oil sandwich plate.

Definately a thermostat, oil too cool does not circulate properly, and can be as harmful as the oil being too hot.

Absolute madness to overcool oil, so damaging to an engine.
Chap I know who does historic sprints/hillclimbs recently commented it takes an age for the oil to get up to temperature despite using a water/oil heat exchanger. But since he has started to take care of oil temps, his engine is lasting MUCH better.

I'd say fit a thermostat!

I've had 2 x 1.6 and have a 3rd 1.9 Peugeot 205 GTI that have oil temperature gauges on the dashboard. It's really quite surprising how long it takes for the oil to reach normal operating temperature. The 1.6 GTIs didn't have oil coolers the 1.9 does.

If you think about a wet sump setup the oil spends the majority of its time in the sump below the engine where it gets plenty of cooling airflow.

For most cars/engines you have to be working them really quite hard (track work) for an oil cooler to come into play.