Guys..
what I meant was, coolant is fine when it's just coolant, but where there's a head gasket leak that is where contaminants gets mixed in - anything such as combustion blow by, oils etc. This causes an acidic reaction between that, the coolant and the alloy and the gasket starts to disappear and the cylinder head metal becomes pitted (which is my old school expression of the leak eating the head), it just gets down hill & worse.
Even some coolants don't mix eg: Toyota (red) coolant and regular (green) coolant together becomes mega corosive, that can wreck a head gasket or radiator and worse.
I don't know if it's a known problem with the 2.8 but cars that have a known head tension problem like the older dattos and later Saab Turbos (same boat) it pays to check the tension of the head bolts at major services.
Re-tensioning head bolts is only a no-no with engines that have the head bolt threads that go all the way into the water jackets, old holden red & blue motors, stuff from that vintage.
off topic - Even had a later engine, commodore ecotec V6 that had to have stud sealant on the water pump bolts, next time you look at a commi V6 that's had a new water pump and often you will see a little stain coming out between one of the bolts and the pump body. Yep you get inside one of those and it's very old school, even the bolts are UNC not metric.