Holden recommends 5W 30 for the Alloytec.
Engine oils come in varying grades, mainly depending on what the engine does and the age. Engines now are much more compact and have smaller gaps between components. Oil pressure does not tell you if the pump is working, it tells you the resistance of oil flow between the components. Putting thicker oil in an engine designed for thinner oil means the oil won't flow the way it should through the engine. More modern engines are designed with smaller clearances between internal components, and require the thinner oils. Using thicker oils in an engine like the Alloytec means the oil pressure will be too high, and you can do damage to the oil pump, other internal components, and gaskets that have the increased pressure may blow.
For all the people that recommend using thicker oils - In most cases, the people who write the manuals and design the engines are much smarter than you or I, so listen to them! Lol.