Just a few months ago, our landlord informed us he is selling the house we rent in. We've rented for 2 years. We recently filled the oil tank , which cost is about 1300$. There is almost a full tank left, is he responsible for paying us for it? We are moving out next week. There was about 1/8 of a tank when we moved in.

Do you have a lease?

Were you responsible for filling the tank? How much notice were you given that the house was being sold? Why are you moving?

Usually, when a house is sold, the buyer pays for what is in the tank. You may have to wait until the closing to get reimbursed.

We have a month to month lease. We were given 2 months to vacate. And yes, filling the oil was out responsibility.

I'm wondering why you put so much oil in the tank in April, having known for quite a while that he was selling and you were moving?
A new owner pays the seller for what is in the tank, usually by filling the tank and paying for a full tank. A renter, I guess, would expect the landlord to be reimbursed, minus the amount there when he moved in, and if he doesn't, he would sue the landlord.
You don't give us exact dates of when you were notified and when you filled the tank.
The 'reasonable' thing to do is sit down politely with the landlord to work this out, and come back here if he refuses to give you a dime.
I suppose that if he refuses, you could refuse to vacate, and it might take him a month or two to evict you, and he might lose the sale of the house... in other words, you have him over a bigger barrel than $1000 worth of oil.
You can also have the oil removed for a refund by paying a smallish fee.

I missed that you said he informed you a couple of months ago. But that raises the question that Joy asked, why would you spend so much to fill the tank when you knew you were moving.

But the answer still stands, normally the buyer reimburses the seller for oil in the tank. So you need to work this out with the landlord.

We refilled in January, not April. About a month before we were notified.

OK, in that case, it shouldn't be almost full. I would suspect, its no more than half full. But you can have the oil company come and take a measurement.