A locking cap on an oil tank is no deterrent to a determined thief. Being plastic it's a 30 second job to drill or punch a hole in it. If they're going to be brazen enough to arrive in something suitable for stealing your oil then having a drill or hacksaw with them to saw the whole top off will be a pretty sure bet.
They're going to get the oil in any case so why sacrifice the tank too. On the other hand I can see the insurance company paying out for the oil and a new tank if it's been damaged in the act but washing their hands of you if you've deliberately left it unlocked.
It's a bit like having to close the shutters when you go away for more than 24 hours effectively giving the would be burglars the green light to come and help themselves !
BTW, had 1189 litres delivered today @.90 = 1080.08 euros, and that's with a coincident delivery of 1000 litres to my neighbour to get the aggregate price
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