On Fungible Goods (al-mithliyyāt)

Answer—and Allah is the granter of success: Oils are fungible goods, and motor oils are of a single genus. On that basis, both surplus (excess-for-equal) and deferment are prohibited in them.
As for sales of that kind that have already occurred: if they took place out of ignorance of the prohibition, then there is no sin in that, and the transaction is ruled valid. This is because oil is not among the specifically texted categories upon which the scholars unanimously agreed to prohibit both surplus and deferment; rather, it is from matters analogized to the texted categories, and the scholars did not agree upon the prohibition of what is analogized thereto; rather, there is disagreement about it. The scholars have established that if a layperson does something believing it permissible, and his action coincides with the opinion of a qualified mujtahid, the transaction is ruled valid. Allah Most High said: “So whoever has received an admonition from his Lord and desists may have what is past …” [Al-Baqarah:275]. This is when the composition of the two counter-values is the same; but if they differ in composition, then surplus between them is permissible.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2