I've copied the following from the Booka Track site; I seem to remember reading this somewhere else as well:
"Engine lubrication - Oil temperature
"With the track day season in full swing and warmer weather here, we have received a large number of queries from various quarters regarding acceptable oil temperatures on k series Supersport cars and Superlights.
We would expect the oil temperature to reach in excess of 100c when driven enthusiastically, but not reach the red.
What has become apparent is that the single most important factor affecting the temperature is the fuel being used. We believe that the quality of 95RON unleaded has detiorated recently, possibly as a side affect of price competition, and this is causing poor combustion at sustained high rpm. This is generally not felt by the driver.
Experience has shown that cars run on Super Unleaded or other high octane fuels (such as Shell Optimax) do not experience this problem. In tests, the oil temperature has been significantly lower under hard use (though it will be similar in general road driving). Note: Whilst our guage only reads to 120c, the engine is rated to a much higher temperature (as is any good quality oil).
Therefore, to safeguard your engine from lubrication related problems, Caterham Cars strongly recommend the use of Super Unleaded or equivilant fuel for track days and racing." - Simon Lambert"
Coming back to this argument of tuning the engine (ecu) for higher octane fuel: it's all down to the ignition advance mapping. Use higher octane & the ignition can sustain more advance. Vice versa with lower octane.
So if you're inadvertantly using fuel of a lower octane number & give it some stick, poor combustion may occur resulting in higher oil temperatures etc. Not to mention what any associated detonation may do to the engine.
From what I gather Optimax is a fuel produced to a standard rather than a budget. So it has a proper dollop of detergent & is 98RON. incidentally Super Unleaded in the UK is 97RON but in France it's 98RON.
Mick