A couple of seconds maybe 2-3. I went by the book and put in a fully synthetic 5w/40 I think and it was Valvoline. I have found that after the engine turns over if I put the air con on it seems to not be so bad....
If the noise you hear changes when you have the aircon on then I suspect what you are hearing might be external to the engine and not necessarily internal. External noise can sound like it is coming from inside the engine so I would check carefully on start up with the bonnet open and someone starting the engine for you testing both with the AC on and then off. For example a bad serpentine belt squeal on a cold start will sound terrible and will not necessarily be continuous(but often is) and can last only a few seconds or it may go longer until the belt warms. If there is a noisy bearing in one of the external pulleys that only makes a racket on cold start up (for now), that could be something to eliminate as the cause of an external noise although when these pulley bearings start to become noisy the noise is normally continuous.
From what you have posted it does not look like the Holden mechanics made any effort to detect and isolate the noise for you but just changed parts to get some workshop revenue from you. In particular I cannot fathom why, if the engine noise is internal, changing an "oil pressure switch" could actually do anything to fix a cold start engine noise(and it did not of course). The OP "switch" is only there to trigger a low oil pressure warning to the dash and probably a signal to the ECU. It does not do anything to affect oil lubrication to engine parts. Oil flow is run from the engine's internal oil pump, oil pressure is generated by resistance to flow and excess oil pressure is dealt with by the oil pressure relief valve.
By the way, check your owner's manual again on the optimum oil spec for your engine. You mentioned 5w-40 is what you are using. As in the later V6 engines, I understood the preferred oil was a 5w-30 for your 2009 engine. If that is the case I would not be using a 40 weight oil.
