This is essentially the legal side of things in a nutshell.
I think whether the fault was there at the point of sale is a moot point here, something like valve stems failing doesn’t happen overnight and could have been failing before, during and after the POS. A judge could easily be convinced the fault was there at the POS.
So that leaves you in a position where you sold the car with a fault, whether you did or didn’t, is academic. It can’t be proven.
What matters here is whether the consumer put 2000 miles on and then complained of the fault or whether they put say 500 miles on and then complained only to put another 1500 miles on. The difference being in the first instance you could deduct a thousand pounds off for usage, the second you could deduct usage and a bit more for the fact they have potentially made the car worse by continuing to use it.
There’s nothing in the CRA to define what you can deduct for usage, just that you can, and your offer to buy it back cannot be trade value.
Given what you’ve said, it seems very unlikely that in the time between sale and the complaint, that’s the car would be using oil so much that it became low enough to cause issues. It would need to be an oil leak which although you may argue they should have noticed, it shouldn’t have been leaking oil in the first place.
The answer to your question is yes you can make a deduction for the 2,000 miles covered but i’d also be offering them a couple of hundred quid over book price on the basis of them driving the car and possibly causing more damage. If they did drive it on after the complaint.