Mathew Pollard wrote:

"the books I have on engines always say the ideal oil temp is 109deg C".

I agree there is an ideal oil temperature but where do you measure it?

Is this measured in the oil pan, after it has been heated by the engine,
or at the oil filter, after the oil cooler and immediately before it
enters the oil galleys?

I have measured oil temperature by strapping a thermocouple to the oil
filter under a piece of foam insulation and it works ok.   I haven't
compared the temperatures to the oil pan temperatures.  I think Frank
Grunthaner recently made reference to the efficiency of the oil-to-water
oil cooler in an I-4 engine so I believe he might have made these
measurements.

At least in the VW world, measurements are usually made in the pan, on
the dipstick, or in the oil filter adapter (for I-4 engines).  All these
are before the cooler.  I wonder how much the temperature drops in the
oil cooler?

Let us know what you find out, Matthew.