I've read this too many times and would like to see what some of the "experts" say. Over the years by some very reliable sources, I've been told that you can't always get what you desire just by mixing two or more items at what you believe to be the right amounts. The best example I can think of was being warned away from 10W40 oil back in the 80's because it had twice the viscosity indexers as 10W30, and that was bad. So what happens when you mix say 5W20 with 15W40? Or how about 3 quarts of 10W30 with 2 quarts of 30W?