This is like having a JD Powers Initial Quality survey vs a 3 year Quality Survey. Sure, at first, the oil meets the spec, works great, etc. But as time goes by, the flaws in the oil are exposed. Conventional oil simply does not hold up over time, even in naturally aspirated engines. The oil molecules are not a uniform size, they shear more easily, and so the additive packs, which is what helps an oil meet a spec, get used up more quickly. Won't see much call for 7,500 mile OCs on conventional oil, even on modern naturally aspirated engines.
Here we are talking about direct injected turbocharged engine technology. High compression, reliance on the oil to cool the engine (which includes injection under the piston skirt) as well as the turbos (same supply), need for oil to flow through miniature passages in the block and timing components, provide a protective barrier in high g situations even if there is temporary oil starvation, fuel economy, etc. Many shoes to be filled by just one oil.
And really, how much cheaper is conventional oil than full synthetic these days? Much as I might not like WalMart for displacing mom&pop operations, you cannot argue with their pricing on motor oil!
Any way you look at it, using conventional oil in GTDI engines makes you pennywise but pound foolish, IMHO.
