The can holds 1 1/2 cups of syrup. That's 12 ounces, not 16. Try measuring just syrup weight, not the entire can.
I store the Hershey's bottled syrup in the fridge; it always comes out thick. I have not had a problem making the brownies with the bottled syrup. I only bake with Land O Lakes salted butter, and in our minds the butter adds to the richness of the brownie.