I actually asked my oil distributor the very same question as it would make life much easier. Apparently however, the reason is because the plastic must be opaque or else UV rays entering the tank, turn the fuel to sludgeI wonder why they don't make oil tanks with a see-through strip on the side of them so you could actually see how much oil was left.
This is a simple matter of bleeding the oil line beside the boiler until all the air bubbles out. Some oil delivery men will even do it for you if you ask them.The plumber must be laughing all the way to the bank!I've had tenants who didn't do it in time and it caused an airlock - resulting in calling a plumber.
Put a pipe into the tank, suck until the oil reaches yourmouth and put pipe into you container! It'll drain merrily away until you have as much as you want!Can't figure out why you'd need a lock on the tank though - how do people steal oil out of a tank from the back of the house?