If you purchased the oil/filter w/ a credit card, you may be able to obtain a copy of the receipt through whoever your card is with or possibly the store itself. IIRC, they can obtain a copy when someone contests a charge.
So, if your dealer is saying it's not covered you can escalate to Acura Client Relations (ACR). If ACR is telling you it's not covered you can request a case review from the National Center for Dispute Settlement (NCDS).
I would make sure to note any discussion in these forums, and elsewhere, regarding connecting rod bearing failure such as this:
If you've had this failure, comment w/ your model year, trim level, SH-AWD or front wheel drive, mileage on the engine and oil at the time of the failure. Include the affected cylinder, if you know which one it was. If this has occurred more than once in the same vehicle, include how many times...
This likely would have happened regardless, but yeah the warranty does require receipts.