What must a repentant person who mixed cheap oil with expensive oil do to atone for his sin?

The rights of people are not waived by repentance. Therefore, you must seek forgiveness from those you defrauded or return their rights to them. If you cannot identify them, then you must give in charity the equivalent value of what you defrauded them of. Strive to ascertain the amount of the fraud by determining the difference in price between the types of oil during the period when the fraud occurred, then donate that amount on their behalf. If you do not possess the money now, it remains a debt upon you until you are able to pay it. If Allah takes your life before repayment and you were sincere in your intention, then Allah will pay it on your behalf, in accordance with the saying of the Prophet, peace and blessings be upon him: "Whoever takes people's money with the intention of repaying it, Allah will repay it on his behalf; and whoever takes it with the intention of destroying it, Allah will destroy him."