http://www.gesilicones.com/gesilwizards/blending/fluid2.html?SMSESSION=NOHere is an interesting little web app that someone just pointed me to. This calculator will allow you to input the viscosity of two liquids, and the target viscosity, and it will tell you what percentage of each to mix.
As an example, let's say you want some 35wt shock oil, but you only have a bottle of 50 and 20. Input the numbers, and you get..

Would also work for diff lube and well I guess anything..