That is not quite true. The typical used oil sample, they cannot tell if it is a 5w40 or a 15w40. No, they cannot tell the base oil, only the CSt viscosity at 100c. And even then, a 40w (conventional, blend, or full synthetic) falls within a broad range of viscosity that is the standard for meeting 40w classification. To determine the actual base stock requires testing far beyond what is cost effective even for the OEM to waste time on. They can determine the basic makeup of the additive package in the oil, which makes up to 20% of any motor oil. And the add pack makes more of a difference than the base oil.
And CNH doesn't blend their own oil. A third party does it and within a given spec that meets the classification desired, the differences are minor between OEM branded oil and name brand you get off the shelf. One really has to give them a reason to be suspicious to even go this far to test the oil.
One probably should keep receipts. But, it is an extremely rare situation the would cause the OEM or dealer to want to see them. Like if your oil was really becoming sludge. Something that looks obvious that you were not changing the oil.