Why you can't mix different motor oils (this is highly technical!)

[Carrameow]

I have read where you aren't supposed to mix synthetic and natural motor oil or even different brands and viscosities as this will cause inordinate engine wear and damage. I hereby offer a rigorous scientific proof as to why this is so:
(a) When i was kid i drank too much soda pop on a 95 degree day and I got a bad stomach ache. I was told solemnly by my mother ( who secretly hated soda pop) that this was the result of drinking 7 UP followed by drinking Coke and that I would get stomach damage and stunted growth because of this
(b) Later on when i was a teen I was told you should not beer of any kind with taco's or burritoes as this would result in your producing a sound similar to a diesel engine just when you started to get the courage to get out on the dance floor and do Disco (remember Disco in the 80's).Of couse this courage came from 6 bottles of beer and the tacos and burrito's too...

A was false but B was true..

Finally to the point: I doubt it makes any diffrenece whether you mix Mobil 1 with Pep Boys 30W, the only thing that matters is you have enuff of the mixture in your crankcase or transmission. The truth about mixing oil? This is an old wive's tale. As long as you have enuff, you are Fine