on my car the oil cooler is built into the radiator, which also helps heat the tranny oil up to operating temp faster.
i thought the 540 did have an external cooler, but it was in a different location. you could find a used radiator with the oil cooler attatched, remove the cooler (is that possible?), and hook it up that way.
Alusil, Dinan DME | ITG Air Filter | Eibach/Bilstein HD | 26/20 Swaybars | Iridium plugs | Depo/ProLumen HID | Optima batt. | no AC | Stoptech brake lines, Frozen rotors, brass bushings, Superblue