I recommend draining the oil and examining it for coolant in the oil. The coolant is going somewhere. If you can't find it anywhere externally, then the only other place it could go is in the oil (or disipate in the combustion process).
As for longevity.... I replaced my upper and lower gaskets at 77K last summer. At 96K (this past winter), I noticed the upper developed an external leak because the gasket didn't seat up. I'm now at 116K and it'* still oozing out. It hasn't been bad enough yet rip the old upper off, but I will do it this fall. When I do it, I'll go after the lower gaskets again, too.
Gotta love GM'* wonderful design.