Oil

Background

My Pitts has always had high oil temperature. I increased the oil cooler size, went from a 3" to 4" pickup scat duct off the #3 baffle. Not much improvement. Oil would frequently run up against the 240 deg redline.

Synthetic Oil Works

I tried Redline 15W50 and Race 50wt synthetic oil which successfully reduced oil temps by 10+ degrees right out of the chute. The shear strength and high temperature nature of oils like Redline make them a good choice. The flip side is poor corrosion protection and high cost. I always took the middle road of Aeroshell 15w50 semi-synthetic, and raced with Redline. Then one day I tried Amsoil synthetic 20W-50 but that caused increased cylinder head temperatures. Then I switched to Exxon semisynthetic and I lost 50 rpm on the top end. Sorry for rambling. Here is my recommendation: If you regularly bust 240F, use synthetic and keep troubleshooting. If redline isn't a problem, use 50W mineral oil.

Lowering Pressure will Lower Temperatures

I had been running high oil pressures as well. I would run as high as 78 psi, a number that was within spec, but unnecessarily high. I pulled my pressure regulator bulb off the case and found 13 washers under the spring. Hmmm, the maximum Lycoming spec was 5 washers. I pulled out 3-4 washers, the pressure dropped 4 psi, and the temperature dropped 10 degrees. Follow-on adjustment has eliminated my oil temperature problems. I run >70 psi in flight, >60 psi at idle (this is apparently unusually high), and >55 inverted. With my oil cooler duct is blocked off about two-thirds, oil temps have yet to reach 212 degrees (potentially a problem).

Will this work for you? I dunno, but it's a fairly harmless exercise to try yourself.

Speculation

I'm sure someone in Lycoming can explain this behavior clearly, and if I get around to it I will ask them. It is a behavior that has been correllated by several people from the automotive racing world. Here's my opinion: Oil flow under certain conditions of pressure, viscosity, velocity, and Reynolds number (a function of oil passageway length and diameter), will go turbulent. Turbulent flow reduces velocity, increases cycle time, and increases temperature. The related hypothesis is that spray orifices in an engine (eg cooling under piston dome) are designed to spray nicely at a certain pressure. Above that pressure, flow can degrade into a sputtery mess, thus providing less efficient beat transfer.

Other Valid Explanations for High Oil Temps

Not enough cooler for the job. Check out what your akro cohorts are using. In my experience the stock Stewart Warner 8-row cooler is not enough for a pumped O-360 running in the California summer.
Clogged cooler
Damaged cooler bypass valve: This valve should only open in the event of overpressure resulting from a clogged cooler for example. If the seat, ball, or spring of this valve are damaged, a significant amount of oil could bypass the cooler.
Poor cooler airflow: Get an old airspeed indicator or differential pressure guage. You should be seeing an apreciable pressure drop across the cooler. Another interesting exercise would be to put a pair of cheap Radio Shack digital thermometers across the cooler to measure heat transfer.
Bad vernitherm: I don't have one but maybe yours does.