I tried mixing different weight synthetic oils in my old pickup and noticed that because of the different specific gravities, or the actual weght of the oil, that they would seperate over the course of a few days. So at startup my truck would be sucking in 20W-50 Amsoil which is not what I wanted.
I new that the oils were seperating after sitting a while because the Amsoil 20W-50 Series 2000 oil is red in color. I usually change my vehicles oil when cold so when I went out to drain the oil, bright red was the first to come out the drain, then it turned to a more brownish color. It also made a racket at startup too, so I had an idea that the oils were seperating after sitting.
Just my .02