Congratulations and welcome to the forum. There isn't a lot of conversation on here lately since these trucks haven't been made in some time. I just sold my 2008 outlook and bought a 2015 Enclave. One to do since it is subzero, check inside your oil cap. Is there moisture and or brown stuff in there or is it dry and maybe a film of oil? Yes the valve stems can corrode with all the salt on the roads, I broke one and replaced all 4.
How many miles?