If they didn't tell you what they
did use, how do you know what they
didn't use?
Is there a part number listed on the invoice? That would tell you what they used (or at least what they billed you for). A mate took his car in for service at an Audi dealers, and supplied the oil he wanted them to use. 2 x 4ltrs of Gulf GVX 5w 30. Asked them to leave the surplus in the boot of the car for him. Got a phone call later in the day to say that there'd been a mistake and the service tech didn't know that his car should be on 5w 30 so had filled it with 10w 40. So mate says "How can that happen when I supplied the oil?" Things continued on and the actual story was that the tech had actually filled his car with the oil he supplied, the service manager had then intervened and said it should be long life oil so they drained the correct oil out and put (mistakenly) 10w 40 in. Then phoned him and told him they'd put the wrong stuff in so there'd be a charge for the correct oil, despite the fact that they'd drained out the correct oil to put the wrong stuff in. They then had to redo the service with the correct oil at their own expense.
When he went to pick the car up, there was a half used 1 ltr bottle of an oil which they hadn't been supplied with, hadn't actually used, and wasn't even any of the spec oils they'd made the mistakes with. So my mate was 3 ltrs of top up oil down on the deal, and still doesn't really know what oil they used.