Mix oils to you hearts content. They are absolutely required to be compatable, and are.

Mixing an old SC oil with a newer SJ oil gives you the overall properties of the SD. Mixing a 10w-30 with a 20w-50 does indeed give you roughly a 15w-40. Mix synthetics with regular oils if you wish. Then you can brag about running a synthetic blend.

The only oil mix I wouldn't do is running a non-detergent oil. It does promote sludging and certainly isn't good in a modern engine. Modern being anything made after the early 60's in this case.