Dear Peekskill Tenant:
New York State is very permissive about terms that landlords and tenants may agree upon when crafting a lease for a private house. Very few state-wide statutes override lease agreements and there is no law that would invalidate or render void the lease you had on public policy grounds.
The landlord like you may forget about the lease provision and carry out her refund to you for the oil remaining in the tank. She will know how much you left behind when the tank is filled on the next delivery. Of course she may try to figure out the value of the oil from your last fill to her fill up, and may check to see if the lease provided a method for calculating the value of the oil left in the tank. Of course when she does so, she will see there was no method to make the calculation because her tenant had to leave a full tank.
I don't know where this is going to get you. You are likely correct that this will come out badly if she believes you tried to take advantage. On the other hand, if she never discovers her mistake how will you feel about this?
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.
If it is in the lease and signed by you then it is enforceable.
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The previous answers are spot-on. My thought is this: return half of whatever she reimburses (or only ask for half) and explain it the way you just explained it here.
I may be guessing or not licensed in your state. No atty/client relationship exists.