I had a new boiler installed but it was never skimmed. It has an automatic water feed so I am not sure how to slowly add water to skim it. Would some chemical be better especially if I drained it after the chemical was put in?

I have an above-ground oil tank in my garage. It's a relatively new tank -- manufactured and installed in 2004 (the year we moved into this house). Almost from the very beginning, there has been a stain around the fill and overflow pipes on the top of the tank. And over the years it has slowly grown in size. It doesn't smell, and every time I've asked an oil company guy servicing my boiler about it, they would always say, it's nothing to worry about, normal. But we just had our house appraised and the appraisal wrote in the report that the tank was leaking. So, I'm wondering now -- is this really a leak or something else as the technicians have been telling me all these years? Any ideas? Thanks.