20w50 is what holden used in the v6 s and 5ltrs 20 years ago, i d say when they went to the gen 3, instead of having different grades of oils they put the thinner oil in the v6
increase the capacity and lengthen the service interval to save money for both holden dealers and customers.. thats my theory anyway
the vs has gulf western in her and picked up 2 containers for $9 each for 4ltrs 9 months ago.... its $18 for 4ltrs at the moment and i still got a couple of ltrs left over
about $25 to do a oil change with filter and i have got plenty of sump plug washers