Others may gasp, but I don't see the big deal in mixing oils. It's certainly not as big a deal as mixing greases, which is a no-no. You seem to be conscientious about oil maintenance which is what really matters. The issues of weights, mixing, brands, etc., have only secondary importance.
I once looked for something published on oil shelf life and found it on the Mobil oil web site. Mobil assures 10 years shelf life for unopened bottles, but I can't remember if it's for the synthetic or dino oil.
95 E320 Cabriolet, 131K