I change my oil every 3 meetings (approx 100 miles with practice), and the oil filter (now I know where to get them from) every time. As Thunder938 has stated it is cheaper than the motor.
For the record, I have got a Hank Scott Oil Cooled XJR1200. Not sure on the temps of the oil, but car is on song at the moment, so something is happy. We run 99 Octane pump gas (higher octane in the UK, but cooler track temps). I use a Fram filter and Castrol Power 1 Racing Oil which is fully synthetic 10w40.
I too think I can go longer between oil / filter changes....but I don't want to take risk of blowing a motor and thinking "I wish I'd changed that oil or changed that filter last night!!!"
Chris