Tremec absorbed BW in the late 90's ('98 or '99) and so the same trans now had Tremec stamped on it.
The same fluid still exists. The synchromeshes are sometimes for some of the other Tremecs (I've seen people report its use in other trannies, like the 3000 series and 500/600's, etc. Check your specific application if using one of those).
Mercon is what I'd use for a WC T5. If you want a synthetic, I've used Redline D4. I wouldnt use motor oil or anything else when there are quality synthetic ATF's out there. If your BRAND NEW trans doesnt shift right with it, there's an issue. And if you used something else, there can be warranty (if applicable) concerns.
EDIT: Mike posted while I was typing.
Good luck.