Your PNL should be the difference between the acquisition price minus the opening price of the position. Acquisition price should reflect what the shareholders at the time of the deal are entitled to. I would rightfully complain if I am treated differently.
If you mean what it corresponds to is that it doesn't correspond to anything. It is a synthetic product created by IG which interpolates between the front and the front + 1 contract. If you mean statistical correlation obviously it will be very correlated to an oil contract.