Like I said earlier, there is no real advantage to what you see as "true" synthetic. Group III is extremely refined, it may technically still be a petroleum product, but it is in no way worse than the much touted synthetics. Group III in Pennzoil Platinum, Valvoline Synpower, Castrol Syntec and Edge, Amsoil XL/SSO, Mobil 1... The only real advantage that PAO adds is better cold flow, and they can acheive that by adding a small percentage, 10% max, of PAO to a Group III blended oil.
Also, the improvement in gas mileage in your "Camry engine" car, is more likely due to the placebo effect, and weather conditions. I've never seen, or heard, of changing oil really making a difference.