What the issue boils down to is the credibility of the company claiming they accidentally filled the wrong tank.
If a local reputable company contacts me and says they accidentally filled my tank and I ask some reasonable questions which they deal with to my satisfaction, I would be inclined to take them at their word and allow them to take back the oil.
It’s all about trust.
In the OPs case, the company is unknown to him/her, not local, unduly hostile to reasonable queries and has sent a registered letter with a false claim that they’re entitled to enter into the OP’s premises to take back the oil.
If the company was genuine, one would think that they would adopt a more sanguine approach and offer as much detail as to how things went wrong in support of their claim to having made a mistake.
For me, the trust went out the window with the attitude, hostility and bogus legal letter.