I've got an interior 275 gallon heating oil tank of indeterminate age (though probably not more than 25 years old). The heating system gets the standard annual maintenance from my oil supplier (cleaning, filter, screens), including the addition of a pint or so of "acetene (A) tank treatment & fuel conditioner" / sudge dispersant & etc. Though I can't pick out a manufacturing date on the boiler, the manufacturer's service literature is present and has 1976 dates. The burner tested at 83% efficiency last summer, down from 85% two years prior. The burner's transfer coil and control unit have been replaced within the past two years, and the flow control valve shows evidence of some slight leakage (and I decided to defer the $450 repair). We use about 950 gallons of oil annually (in eastern PA).
My oil supplier offers a "tank cleaning" at $250. Being flush with cash at that time, I had them do this in 2000. Basically, they drained all of the remaining oil and did whatever other magic they do as part of that work. On at least one occassion since that time (although we're on automatic delivery) we actually ran out of oil, and so the tank was presumedly thoroughly drained.
Here's my question:
Is there enough value associated with cleaning out the tank as to make it worth doing periodically? And at what interval? Is the "tank treatment" fluid likely to be sufficient to maintain the interior of the tank in reasonably good condition?
Thanks in advance for your observations and advice.