I think you'd be fine with a good 10W40. Staying on the lighter side might help to facilitate with the seating of the rings. (Others on here will certainly offer opinions as to favorite/desired wieghts and/or brands, so I'll leave that issue alone from here on.) When you DO get to a respectable mileage/runtime on this engine and decide to thrash it hard on the strip- you can go heavier weight (or for the June-August extreme heat months, as I notice you are in Dallas). Bear in mind that WHILE this ring-seating process will be occuring, it's not unusual to have some usage of oil, and the thinner the weight-the faster it can be used.
Anyways, it's your call as to wether or not to just focus on the one exhaust valve, or do a more complete inspection and replacement procedure, but EITHER WAY- you've got to find out WHY your stem is getting the accumulation on it. It just boils down to how involved you want your inspection and analysis process to be.I mention pulling the heads and disassembling the valvetrain ONLY because of the fact that you will know for SURE what conditions of all valves and guides and supporting hardware are. PS: When you install VC baffle(s)- be very certain to use a good threadlocker and clean thread surfaces to keep screws/bolts from vibrating loose and falling into the valvetrain. Good luck on all aspects.-Jim