What kind of gear oil for 8 3/4? Why not synthetic gear oil?

richmond says not to use synthetics because they don't mix with friction modifiers well. i had a long discussion with a guy at richmond about a 55 belair i was doing that had 1 of their trannys. he had problems with synthetics not mixing with the additive and noises coming out of the rear until you drove it a bit. i use lucas and haven't had a problem yet.