If I care which sats or which $20 I'm receiving in trade, then they are not fungible. If I don't care, then they are.

The rational buyer cares about which path he pays and the rational seller cares about from which channel he receives.

it's fungible because I don't care which barrel I receive
Fungibility is indifference and indifference is a property of individuals, not a property of goods.

Fungibility is not subjective. Oil is fungible because the hydrocarbons are identical, not because of your apathy.