Here is an update. I emailed Blackstone labs and explained the issue to the tech who handled my inquiry. Their used oil analysis might or might not come up with useful findings. If this is something like base stock but no additives, their testing would likely recognize that. Anyway, I felt it was worth it to go for it, and I ordered a test kit, which arrived today. I mailed it today, in time for pickup today from that mailbox. So they might have it before Memorial Day. I will share the results when I have them.