[Hags86]

The answer is the thinnest possible oil that gives sufficient oil pressure. Get an oil pressure gauge.

This is a scatter plot of my car of oil pressures (Y axis) vs revs (X axis) where the colour is oil temp. The data is a shakedown run on my turbo setup, maybe 5 or 6 laps with just two hot laps. Stock bottom end, 10W-40 synthetic oil, 240rwkW on e85 with an oil cooler.

As you can see oil pressure drops away steeply as oil temps increase. The rule of thumb is you want 10psi of oil pressure per 1000 rpm. E.g. 70psi at 7000rpm. The more power the engine makes the more oil pressure you need.

What this data tells me is 10W-40 is barely thick enough. The cross is showing sample of 52.9psi at 7335rpm with oil temps of 115C. This is too low a pressure.

I think an xw-30 will be too thin at elevated temperatures on a track after a few laps. A 40 weight is the minimum on a stock bottom end. I'd have no hesitation running a 50 weight if you were happy to do an oil change before and after track days as you mentioned.

An oil cooler and oil pressure gauge are both essential items if you are tracking this with a turbo.