There is a hell of a lot more to oil these days then most know, theres your API rating, SAE rating ect. but not all oils with the same rating are as good as each other (cough cough gulf western for one). Im pretty sure the latest and greatest rating for petrol is SM and for diesel its CJ-4. SAE is generally your thickness, or 'viscosity' , 15w 40 is probably one of the most common, W meaning winter or cold weather. Different oil companies all have slightly different levels of additives, detergents ect thats why some brands of oil will burn excessive amount in one engine, and then when you switch to another oil of the same grade it doesnt. I would recommend whatever the manual recommends as the big companies such as Holden get an oil company to develop an oil for their engine to Holdens specification, or they might select and existing oil that meets or surpasses their needs.
So i say whatever the manual says is the best suited, given the manual is about 10 yrs old the oil may be out of date and you should consider a newer oil with the SM rating and same SAE as the original. Im no expert on this subject.