I'm curious, since I know that Australia doesn't sell much 5w30 or 10w30 oil, what does the owner's manual for the LS1 Holden recommend to use? 20w50?

 

10w30 if not available SG, SH or SJ oil with a viscosity of 20w50 or 15Ww0.

HSV are Mobile 1 from Factory

 

A lot of people try to tell me that GM designed these engines with 5w30 oil in mind, but I believe the only reason GM specifies 5w30 as the preferred oil in the US is due to the CAFE fuel economy regulations. That's why in Australia they can say to use a thicker oil. I personally wouldn't run a 20w50 though, but wouldn't have a problem with a 15w40 in hot weather.