Not Castrol for a start.
Why?
Because I'd rather buy a fully synthetic oil instead.
Bull$hit - utter UTTER BULL$HIT.
If Castrol state their oils are fully synthetic - then they WILL be fully synthetic!
You arn't a Mobil, or other spam oil fanboy?
It's really not bullsh*t.
The base oil stocks were not fully synthetic, they were hydrocracked mineral oil which is grade 3 oil, not actually fully synthetic. But due to their processing it was deemed (under industry law) to have had enough done to it to class it as synthetic.
But it's VERY well documented online.
Your last reply was a bit out of order - doncha think?