I've noticed a trend as of late with High Mileage synthetic oils. I know Mobil One has had a synthetic High mileage for awhile now, but now its Valvoline, Castrol and Pennzoil. What is the benefit of a full syn High Mileage, Vs just running regular synthetic? I noticed only Valvoline and The new (to me at least) edge high mileage are DEXOS rated? Why is that?