It should be noted that the weak spot in the Omega cooling system has nothing to do with the coolant.
The HBV should be treated as a service item. Even genuine ones have been known to fail in short order, ie within two years, and when* it fails, it will dump the coolant.
The problem is not knowing what combination of neglect has previously been used to top up the system following a slow leak (reseevoir/cap) or a catastrophic one (HBV).
The moment incorrect fluids such as tap or mineral water or incompatible coolant are used, the system degrades from the inside out.
It isn't possible for one coolant to meet every manufacturers specifications. And given that coolant changes are a service item, I would suggest sticking with Dexcool as described by GM.
*If isn't an appropriate word in this case as at some point the HBV WILL fail.