It's like you've got a car that pulling to the left and you've seen a TSA saying there's a known problem with the shocks, so you're convinced it's that, while ignoring it could be tracking or the flat tyre, etc. Pulling to 1 side suggests there is a problem, but doesn't pin point what it is... (as a simplistic example)
Without a black and white written statement from a specialist (or Nissan) saying "the engine has this and that wrong and needs a rebuild/replacement" then, realistically, the used dealer can pretty much rub their hands and say that they're not a specialist and as far as they know it's characteristic of that car at that age, with typical wear and tear, and thus it was correctly advertised. For them to refund or replace it needs to be broken - yes, something doesn't sound right, but (as far as I've seen) you still do not have a statement saying x has failed, so you cannot demand the used dealer to replace the entire block.
Without a 100% positive statement that x is broken then you almost need to rely on the used car dealer being friendly and offering to help. This obviously won't happen if you poke him with a stick saying he's got a bad reputation for an incident that is 100% not linked with yours. As others have said, you've probably screwed yourself royally by even mentioning it. If they're not a specialist of this car or have lots of them on their forecourt then (unless it's crunching away, which you would have noticed when you test drove/collected it), how do they know the engine note is off? Surely you don't expect them to test drive all their cars for 500-1000 miles to check oil usage? So, they probably don't believe/know there is an issue with the car - which is why everything has to be done in a friendly way without finger pointing and name calling...