I am not sure wether this has been discussed before, but here goes.
Birds have a very thin layer of skin under their feathers, and therefore rely on them to keep warm, and, of course, to fly. For this reason Brrds need to look after their feathers very well. Many Birds have preen (or "uropygial")glands which secrete an oil that cleans the feathers, and waterproofs them.
Since raptors and other Maniraptorians had feathers, it is likely that they had some sort of preen gland, however some birds merely crush ants and use the juices to clean their feathers of parasites, so it is questionable. in Birds the preen gland is locaed at the base of the tail, therefore it is also likely that, if they had it, it was there. However, the Dinosaur would have had to be able to twist it's body so that at least the front teeth/part of the beak could touch the gland, and therefore retrieve the oil.The question is, would some Maniraptorians be pyshically capable of doing this? Another question is, did dinosaurs with protofeathers need the oil, and did they have such a gland?
I am not sure myself, but does anyone out there know?