I didn't install an
oil cooler initially, but found that I needed one.
I don't track my car, but I found that sustained periods at freeway speeds (well, maybe a little north of posted speed limits) and in hotter summer temps, the
oil temp would creep up to as high as 125C (257 F) and appeared that it would continue climbing if I didn't back off the revs for a spell.
I agree with the problems with
oil that is too cool, so I installed a thermostat.
Now, sustained freeway speeds (about 3,000 rpms) only gets the oil to between 105C and maybe 110C - a good operating temp - and it doesn't go any higher.
So my vote is put one in if for no other reason than originality and use an oil cooler thermostat. For what it's worth, I used an Earl's sandwich type and pics are in my gallery.
Matt