I understand that oil is fungible under most circumstances. But it is very unusual for a major producer to outlaw exports.
From Wikipedia:
Fungibility is the property of a good or a commodity whose individual units are capable of mutual substitution. That is, it is the property of essences or goods which are “capable of being substituted in place of one another.”
So oil is oil and American crude can be substituted for anyone else’s crude. But how can substitution take place if exports are illegal?
First, as I said, there can be major differences in oil qualities. It is NOT all equally replaceable.
Crude oils have different quality characteristics
https://www.eia.gov/todayinenergy/detail.cfm?id=7110
Secondly, the US export ban has been lifted.