I would advise against simply jumping into a 15k mile OCI on a vehicle with questionable history. I wouldn't do it myself, and I'd advise against it, but do what you think is right.
In fact, go get two more 5-quart bottles of M1 0w-20 EP and two Fram Ultra filters, then claim the $28 in rebates currently available on them. If the engine doesn't grenade on you, then you'll have another 30k miles worth of oil and filters, and the $28 you get back in rebates will pay for another UOA.
But I wish to repeat, I advise against jumping into a 15k mile OCI on a vehicle you don't know the history of, and haven't done a UOA on to gauge how far it can go on an OCI.