I think you need to correct your data by using the degree days per month or year to see what you are spending relative to the out door temperature. If one year has 20% more or less degree days your energy use should reflect this. Your oil company should have this data. If they do not they are less than professional though there are plenty that deliver at fixed times to avoid buying the equipment to generate the data. Possibly there are inexpensive weather stations that will record the data for you.
I can look into that easily. My deliveries of oil are actually based on degree days and the data is posted.
I will see if I have that data going far enough back.
It is a good point too, as last year was so warm, I didn't even use the stove much at all while this year it is just constantly going.