Here are a few idea that you can look for when you see the car in person.
I am unsure why this would only "occur over 5k rpms" but I can tell you what are common problems in this car.
Check the oil cooler lines, feed and return. These ALWAYS dry rot and would make sense at higher rpms (higher oil pressures) that this could rear its head more often under those conditions.
Distributor O-Rings. Same deal.
Valve Covers. Valve cover bolts are 9 times out of ten finger tight and will always back out with time. Then you have the joy of cleaning out all the oil from your plug valleys and re-sealing the valve covers and adding real bolts that actually hold
Those are the 3 biggies that I suggest that you can tell upon visual inspection. Good luck with your MR2 hunt!