Unfortunately, oil coolers tend to be specified by the aircraft manufacturer rather than the engine supplier, so any aircraft manufacturer using the PT6A 65B would probably be a good starting point to contact.
Excuse me for my late reply. I was sure about that (the airplane manufacturer must specify the oil cooler) but I hoped that, with a little luck, somebody could give me some references.
In fact, it was my mistake, because I didn't specified nothing about what kind of airplane we talk (or even a heat quantity to be exchanged, for example a value around 1100 Btu/min and an oil flow rate about 120 lb/min).
Anyway, I'm interested in something similar with Beechcraft 1900 (or another airplane in this class). Unfortunatlly, I didn't find nothing convenient on that web site which you mention it.