And until just the last year, Mobil 1 was only a synthetic blend.
The problem with a synthetic blend is the difficulty in controlling the base stock and producing a reliable oil that has consistent performance.. But hey, that was better than some others I can think of..
I guess you need to define your terms. From what I understand Mobil 1 has always been a full synthetic. Originally it was full group IV PAO and later became a blend of group IV and group V which it still is today.
If you use Castrol's definition of synthetic, the newest Mobil conventional oil would qualify, both use full group III base stocks. 
