Some great advice on this thread, so rather than re-hash it all, will just add a couple of general comments!
If you don't have an oil temperature gauge, (or a logger recording temps, even if not displayed in real time), you really have no idea if you have a true issue or not, in 90% of cases.
Unless an out and out competition or equivalent track car, that only sees careful warm up/cool down and hard track use, if you do fit an oil/air type cooler, you need a thermostat too*.
If you're more of a balanced year round road user, you probably, for most common Westfield engines DON'T need a cooler. Even on the engines dogma tells you, you will.
If you like to mix year round driving with very hard road driving on, oh, lets say Alpine style roads , or perhaps a generous number of track days as well, without actually living at the track, then seriously consider an oil/water style cooler. Some of the common Westfield engines, Honda F20, Duratec etc actually come with the OEM manufacturers version already, in fact. Though subject to what your oil temp gauge is telling you, this might not be enough. In which case a bigger, Mocal or similar style cooler, (which may also mean a slightly bigger or more efficient rad, too), is the thing to go for.
The Oil/water type cooler, have the advantage, that they won't over cool the oil, in fact, they'll actually look after it better, helping it get up to a minimum operating temperature faster. Great for winter/colder days, or even just getting the engines T's and P's up to working levels faster, and more mechanically sympathetically.
The oil/air type cooler, has some benefits for pure track cars, efficiency wise, and oil pressure wise, (potentially), BUT without a thermostat, will over cool the oil when not driving hard, which is also bad for the engine/oil. A thermostat will help, here, true, just be aware, that the stats for oil systems are usually different to those like you have in the coolant system. To avoid pressure drop catastrophes, they will frequently still flow more oil then you might think, through them, even in the "closed" position. So with this type cooler, physically covering it in colder weather is often best.