Despite what GM shove on the sticker, its not really good for 5 years. In the same way my XJ shouldn't have to wait 16k for an oil change. I generally like to change my 5yr coolants at 2-3yrs, and the older 2yr coolant annually (though the Kawasaki was the last vehicle to use that)
Coolant, like oil, starts to degrade from the second it goes in, and I know you're not daft enough to leave your oil in for 20k, as per GM spec. No offence intended with my earlier remark, but we know from previous posts that the Omega has been at the bottom of the priority pile. Just offering as a suggestion as to why the normally bombproof 3.2 MLS gasket has failed, *IF* it has. I'm not yet convinced it has, as a coolant to cylinder failure would often give massive pressure in coolant, and exhaust fumes in coolant bottle. Personally, I'd be getting my ass in the valley, and pressure the coolant to around 8psi.
As said, not offence intended in any way, and hopefully none has been taken.