It seems to me that "Dexos 1" would have to have a synthetic component just by its very nature to perform better than GL5. Anyway, I have checked out other brands of motor oil and a couple do sell full synthetic "Dexos 1" spec. oil. Mobil 1 & penzoil come to mind. What I don't know is should the Blended synthetic be used in G.M. vehicles as this is what was put in originally. Are you ahead by spending a little more for full synthetic.

The thing with synthetic blends is that there are not standards as to how much has to be synthetic to call it a synthetic blend. Some published that information but a vast majority do not. Shell is the supplier of AC Delco dexos1 oil.

I use AMSOIL 100% synthetic in everything and have been since the 80'*. Its in my 2009 Cadillac CTS and in my 2012 Sierra Denali AWD 6.2L.

Being on GM'* list means nothing to me.....

My take on this, GM is blending dino with synthetic for a reason.
It may offer better shear characteristics and possibly bond well with valve train & seals especially in the VVT engines. What percentage of dino is in there is anyones guess.
If it is a third dino, a cost saving would sill be recognized and less carbon byproducts.
For my older 97 Pk Ave I blend my oils. One quart of High mileage 10w-30 and the rest synthetic. I also add one bottle of Lubegard blue bottle. My valve train quietens down big time with the lubegard bio engine treatment. LUBEGARD BIO-TECH Engine Oil Protectant
In my Jeeps limited slip differential I wont use synthetics. Lucas 85w-140 is in there, I want the gear thick and sticky oil in there. Sometimes I think synthetics become a little thin when real hot.

Mixing Synthetic Motor Oil with other oils will shorten the oil'* life expectancy and reduce the performance benefits.

I am not so sure about that statement. You can gain other performances in ur own blend.
I change out at 3000-3500 miles regardless of what is used.
If you take Mobil 1 syn (GL5) and mix Mobil 1 Super 2000 (high mileage syn blend also GL5) or even just use the super 2000 alone for vehicles that have above 72k miles, according to manufactures specs you would gain :

Helps extend engine life
Contains seal conditioner to help prevent or control leaks and reduce oil consumption
Protects older engines, (probably better burn off protection too.)
Outstanding wear protection under a wide variety of operating conditions
Easy cold starting -pour point -33c
I see no reason why a DIY blend of 50-50 of the same brand of oils in GL5 could offer a better than stock synthetic performance, for engines with more than 72k miles.

There are always those who apply VOODOO decision making and convince themselves that they are making superior decisions by following some self created formula. What they do doesn't hurt anything, but there is no proof that it ever helped either.

Yes that is what I use to think as well.
Until I bought the Park Ave with 81 k miles on it. And treated it to it'* first Mobil 1 full synthetic oil change, and shop noticed that the clicking lifter noises, were very much present.
In front of the experienced shop guy'* I pulled out the synthetic LubeGard Bio engine treatment and said are u ready for a quieter motor..and they went ya right....
300ml blue bottle got added while the engine was running and within 45 seconds the lifter noise completely went away. Smooth lifters with friction now removed is proof enough for me and them. No one believed it. They now buy it by the case load and sell it big time. It sells itself.
Is there something missing in friction modifiers on full syn'*? Is it thin?
Try some bio engine treatment and report back your findings....
You might even be pleasantly surprised the engine/oil is running cooler too.