The pertinent question is: What is the viscosity of the oil at operating temperature. *IF* under track usage, oil temperature gets as high as 280F, then a 20weight is going to be at ~3cSt, while a 15w40 is going to be ~7-8cSt.
At least according to this chart (which I cannot vouch for).
Knowing what the operational oil temperatures are would be required to know what viscosity to run. Without knowing oil temps, I would tend to go at least one range higher in viscosity for track days. Running a 5W30 in these cars even at very cold temperatures isn't going to be as risky as running 0W20 at 250+
I've always just run 10W30 (factory spec) synthetic in the s2000. But in the LS2 RX-7 I run 40wt (vs 5w30 factory spec) in the warmer months when I will be tracking it as a hedge against high operating temperatures.
Really need to get an oil temp gauge though. For all I know I should be running 15W50...