When adding an external oil cooler I can't easily think of a scenario in
which you wouldn't want to include a thermostat in the circuit.  Ideally the
oil gets to optimal temp quickly and stays there.

I have a cooler and thermostat from this site:

http://www.bus-boys.com/

See this page:

http://www.bus-boys.com/oilcoolingremote.html

I got the full setup with the larger cooler, thermostat and filter sandwich
adapter from them and have been very pleased with it for years and thousands
of miles.

No matter what setup you go with a thermostat will help your engine last
longer even if you live in a typically warm climate.  If you live in a warm
climate you could get one of the engine oil heaters (that race teams use or
similar) to keep your oil at operating temp even when you aren't running the
engine, in that case you may not want a thermostat in the oil cooler loop.

Jonathan Poole
'83 AC Westy