I'm not arguing the craziness of the oil thing, but am fairly certain there HAS been occurrences of people having warranty work denied on engine issues because they either A) could not produce receipts they did changes within 6mths/6000 miles; and/or B) they could not show the appropriate SRT Pennzoil Ultra Platinum was used.
You may be in the right and feel it is not your duty to prove negligence, but FCA tells the dealer whether work is going to be covered/reimbursed after the dealer submits the paperwork. If their answer is No, it's not being covered until YOU fight and prove otherwise. So from there, it's on you to get the lawyer, etc. etc.