Is it incumbent upon someone who worked for a company that sells diesel, knowing that it cheats on the quantity, to do something ten years after leaving that job?

The questioner must repent to Allah for assisting Muslims in cheating and for abandoning their counsel, for Allah the Almighty says: "And do not cooperate in sin and aggression," and for the Prophet's (peace be upon him) saying: "Religion is an-Nasiha (sincere advice)." Nothing more than repentance is required of him, unless he was directly involved in the cheating. If that was the case, then he must compensate those he cheated. If he does not know the extent of the cheating or those he cheated, he must strive to estimate it and give it as charity on their behalf.