Well...it seems that Castrol decided to sell Group 3 oil as Full Synthetic...so Mobil decided to see if that was fair, as Group 4 & 5 were really considered as Full Synthetics, as they had to be created chemically...

...whereas Group 3 were pretty much dino juice with extra cracking to refine it better, etc....

...so selling Group 3 was cheaper than selling Group 5...and Mobil1 was a Group 5, being threatened in the marketplace by a Group 3 Castrol labeled as if it were just as good.

The US Gov decided that "Synthetic" as regards to motor oil, has no legal meaning, and therefore...anyone could call pretty much any refined petroleum motor oil "100% Fully Synthetic Motor Oil", etc.

So - Mobil, and a bunch of other oil makers, like Penzoil, Quaker State, etc...dropped their Group 4 and 5 products from the shelves..and replaced them with/Introduced Group 3 motor oils...w/ changing the labels...so as to be cost competitive with each other.

The changes started a few years back...and have now been completed across the mass producer product lines.

There's almost no "real" synthetic oils left from the mass producers.

I think Redline is still a Group 5, and Amsoil is still a Group 4...not sure about Royal Purple...might be still a mix of products that are still Group 3 and 4, etc.

sigh.