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

you might be right, but i try to always stick with a good brand name.

i usually use rotella in my 616 and 617 motors. i currently have the mobil 1 2050 in my 90 350 sdl.

pep boys 30 may be just fine but i will pass except in an emergency.

and just to be safe i go easy on the beer and burritos as well.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I love urban lengends and all the disinformation that goes with them!
('Kinda like getting medical advice from "Urban Outdoorspersons")

"What I believe may be chemical facts"

1.Mineral based lubricants "Breakdown" at much lower temperatures than
synthetics.
2.Mixing "Dino" and "Syn" makes the "Dino" the weak link in the chain as far
as temperature degradation.
3.Since "Syn" is smaller on a molecular scale ,...YOU may experience more
of a tendency for LEAKS.(If you already have a mineral based leak!)

Yikes! Another oil thread....

You wouldn't mix HaggenDazs ice cream with the store brand at Kroger, would you? Even though your stomach wouldn't know the difference?

You wouldn't mix Johnny Walker Blue Label with Ambassador, would you? Again, your stomach couldn't care less.

You wouldn't mix Benjamin Moore paint with the cheapest stuff from Wal-Mart, would you? But, the wall might not know the difference.

So, why would you mix Poop Boys 30 weight with Mobil-1?