Viscosity is related to engine oil temperature. If your engine oil temps are exceeding what the car was designed for, higher viscosity would be a benefit.
For the street or autox, you'll be fine with the 20w, IMO. For sustained track use in a hot climate going up one notch to a 30w is probably a good idea. An even better idea is to use the best oil, regardless of viscosity. If you really care, put some Redline or Amsoil in. That's what I do. For the track, I'd rather use a 20w Redline than 30w cheaper oil.
