Gotcha, thank you for the clarification.
This sounds like it may be a simple communication issue with your customer's QA team. I would have considered oil residues found on web cores to be a contaminant in my last position, simply because it wasn't normally found on them, and our specification for the web didn't identify that that would be an expected thing to see. Is your customer aware that this is a processing aid in your facility, was the expectation clear that this is considered normal? Just finding this on the floor would tell me that something got spilled on it in transit or at your warehouse, hence the risk of contamination.
I would still be hesitant to use this material, because it's an unknown variable. I might have accepted it as a customer had this exchange gone through:
Email from me to jdominguez:
Hi Film Supplier. We've placed the last shipment on hold for what looks like some sort of liquid contamination on the cores. Please schedule return and replace the product. See attached pictures.
From jdominguiez to me:
Thanks for the email, looking at the pictures that staining you see wasn't due to a contamination event, but as part of our normal process. It's canola oil that's used as a process aid to keep your product from creasing during winding. While there is a more apparent stain on this core than normal, this is a common thing to see in our products and does not affect the quality of the material. As for safety, the canola oil is applied at our plant and maintained as a food contact substance. Attached is the SDS/supplier CoA for the canola oil that we use, and some pictures of the process to show how it can happen in our process.
The pictures of your process will help a lot, as well as some examples from other products you make that look similar. They key here is that unless they understand and can document that this isn't evidence of unsanitary handling of the material, they can't use it.