OK thiggy,
I feel my previous reply was a little too flippant. The other comments about this dino oil having a ton of VIIs is right on, but that doesn't necessarily mean it should be tossed.
Rethinking the "blend it" concept a little, I guess what I'd REALLY do in your situation is blend it somewhere between 40-60% with M1 10W-30 or 15W-50. I think we've all agreed that the new 10W-30 & 15W-50 are M1's least VII, most stable (non-thickening/non-thinning) oils. Let's assume that you can expect the GTX to thicken a bit & thin a lot. The advantage to the 10W-30 is that it starts at a syn's 10W (better flow than a dino's 5W) & won't ever drop below a 30 when hot. The advantage to the 15W-50 is that it flows better than a dino's 10W & has a boosted anti-wear pkg in it to offset whatever degradation has occured in the GTX.
Disadvantages? Well, the 10W-30 doesn't have the advertised add'l A/W pkg, & using 15W-50 means it's gonna stay a lot heavier. I really don't think 50wt is a problem for your vehicles/area, but it's obviously not a necessity. (Remember, it's still a 10W when the engine's cold.) I think I'd go with 10W-30 for 5k mi.
It really comes down to your comfort level. I don't think using the GTX will harm either vehicle, & blending it is really just "insurance" if you don't want to toss it. I'm very much pro-extended-drains, so if I got a case free I'd try to make it work for me as much as possible. Also, I wouldn't run it in my newer, still-in-warranty car, but I would in my older, 160k mi car. But are YOU comfortable with that?
I guess I should now add "anal" to my previous self-description.
