My Ion loved ST Synthetic. I ran ST most of its life before switching to Royal Purple for a change or two, then Mobil. (Using QSUD now because it was on sale.)
It consistently got 36 mpg with the ST 5w-30. The only other oil that managed that was Mobil 0w-30. With all other oils, I've gotten between 32-34.
OCIs were whenever the oil monitor said "CHG OIL". Mileage varied wildly, from about 5000 to almost 7000.
I refuse to go to Wal-Mart these days, but I may try out Mag1 synthetic, from Amazon. It appears to be ST on steroids, at around $4.99/qt.