Valvolene and Mobil-1 are both using the Chevron-pioneered hydrocracking technique with group III stocks nowadays (for most M1 products, anyhow). There's not a whole lot of difference.
Now, back in the day, Mobil-1 was a group IV in their entire line and that was stuff to have. That and Amsoil, Redline, Torco, Motul, and a few others.
Purple was always a Group III, btw. Never worth the money, imho; not even close.