.... but then you need to account for wear and tear and the fact that positive displacement superchargers are harder on the bottom end (the rotating assembly) then say your normal ecotec V6.
Personally, I run a 10w-40 or 15w-40 mineral or semi-synthetic. I've used different brands including full synthetics and always found the engine/lifters would get noisy around the 7-8000km after an oil change. Now I run Penrite oils with the high zinc content and no longer get any engine noise. One the down side, it seems to use a little more oil? and I have seen that trend on a few different cars I've changed over to Penrite oils. I'm also in NZ where we have much lower ambient temps so things run a little cooler.
A little experimentation never hurts. Try a 15w-40 if it gets noisy then put in a 20w-50.
Well that is the million dollar question. VS recommended 20w-50, VT series 2 onward 10w-30. I think it has something to do with what was used in the "other" engine in the Commodore in the same era.
I have written a lot on this subject in the past. But I've also started looking at the engine itself. Have a look at the bearing clearances on the V6 motor compared to lets say your average 350 Chev or our humble 304. The V6 runs much tighter bearing clearances and also much higher oil pressure. We measure oil pressure but in reality that is wrong. what is important is oil flow/volume. Pressure just measures resistance to flow. IF you locked up the oil pressure control valve so you would see actual pressure if you changed to a thicker oil you would see more pressure because you are seeing more resistance to flow so in reality a higher pressure would indicate you are actually getting less oil volume and then consider the oil pressure is controlled to prevent excess pressure and you are getting even less oil flow/volume where it is needed.
I think if you want a clue as to what oil ideal for the ecotec/L67 check out what was recommended in the US motors in the southerns states.
As far as I'm aware zinc is detrimental to the cat converters but these modern oils have zinc at lower levels then what you saw pre 80's and are said to be catalytic converter safe. I've been running it now for over 40,000km and have not noticed any issues with the cat. if there was I would expect the converter to start failing/blocking up.
That's awesome with the cats not blocking, I run that oil in the Datsun and have in heavily flat tappet cammed 304/355's thinking if it does a cat or two in it's cheaper then wiping a lobe and putting metal through the motor.
Haven't ran it in a car that see's KMs to see if there were any long term issues.
Definitely agree that pressure is a measure of resistance , wasn't to sure how these things flow though. I've heard I don't have to worry about head drains at 7200 rpm so they must keep a good amount in the block and not over fill the top end.
Just did some digging on American sites in 98 they changed it to 10w30 specifications and 2004 it changed to 5w30 but when originally was developed they had 15w40 cold climate 20w50 for 96 and 97.
I'm wondering if they dropped it for cold start noise , keep it aligned with other motors or an emissions thing maybe.
But seems blanket worldwide viscosity change.
I also just found this , guess I'll be able to know exactly what oil to use after I assemble it LOL
Factory clearances on the ecotec/L67 for VT/VX is .023mm to .053mm or .0009" to .0021" working on the assumption that engine oil operating temp is over 100°C or 220°F the chart above recommends 10w-30 if the bearing clearances are on the loose side but if on the tight side a 5w-20.
Oil viscosity change could also have come from the need to run a thinner oil to reduce parasitic drag to improve fuel economy. Economy is a sales marketing tool. Engine life lasting longer than the warranty period not so much.
The other thing I don't like about our engines is that the oil pressure sensor is right after the filter so the sensor sees best case scenario, after the filter the oil goes through a number of 90° turns before it enters the main gallery. Every 90° turn reduces flow. At least the LS engines got that a bit better with the sensor been on the back of the block and takes it feed from the main galley if I'm not mistaken.