oil level varies and tanks do rust, the rust thru doesnt have to be at
the very bottom either. many likely leave their tank level low in the
summer. so rust might be low many insurance companies wouldnt cover
homeowners insurance for tanks over 25 years old, because of the
environmental clean up costs when a leak occurs let alone the added
fire or health risk.

insurance today doesnt cover knob and tube wiring, visibly bad roofs,
cracked sidewalks etc.