The motor's different Simon (5.0L Ford), but the same issue showed up in mine. Never a problem with coolant temps once the cooling system was done, but when I put the Volvo optional VDO oil temp gauge/sender in, I found I had oil temps consistently in the 250-260F range.

On one hand, that seems awfully high to me. On the other hand, this engine (and hundreds of thousands like it) survived it's first 60K miles with those oil temps and was none the worse for it.

Nevertheless, I bought a Ford Racing oil cooling unit (stacked plate/self-regulating temp design - not fin tube) for the Mustang for about $110. It came with the coil, hoses, fittings, sandwich adapter - everything needed.

I mounted mine at about a 45 degree angle low in front of the opening in the center of my air damn. I built a shield so that hot air coming off the oil cooler DOES NOT re-enter the air stream for the condenser/radiator. I didn't want to turn the radiator into the oil cooler.

At cruise, temps are now in the 210F-215F range. They creep up into the 220-230F range in stop and go. For grins I may buy a used motorcycle cooling fan and put it on the oil cooler to help it function at low speed.

Your set up is of course different, but I'd be happy to send you some pics of the mounting if it would help you.