You can run either, both are the same viscosity at operating temperature. If the car still has warranty, I'd stick with 0w-20. Synthetic will always have the edge over conventional for protecting in high heat. I'd just stick to the 0w myself if it was me.
5W20 used to average slightly higher viscosity than 0W20 but these days they seem simular.
If you want a little higher viscosity why not try one of the thinner 30 grades like Pennz Platinum or QSUD 5W30? Should be be roughly 10-20% higher viscosity than the 20 grades.
You're fine with a good 0W-20 synthetic. If it starts to use some oil then go to a 5W-20 or 0/5W-30. But Mobil 1 0W-20 AFE has a Viscosity, @ 100ºC, cSt (ASTM D445) of 8.7, making it not too far off from some 5W-30's...
It would actually be better to use the 5w-20, it would be more shear stable. Better for your warm climate. Viscocity at temperature will be within specs. I agree and would only use synthetic.
If the M1EP is actually "as advertised" in the SDS, there is your best 20 grade on a shelf!
Real PAO synthetic.
Nothing else from a Major is. Pennzoil GTL are isoparaffin base IIRC.
FORGET the W rating. If you were running el-cheapo 0w20 maybe you would find an HTHS and sludg-ing tendency improvement in another el-cheapo 5w20. But as you are currently running one of the few remaining REAL full synthetics designed for long life Oci, you will just be getting poorer overall performance elsewhere.
HATED the PP GTL 0w20 in the wife's Subaru.