Break in oil(done right) should be straight weight oil without high additives for anti-wear. That is supposed to be run for 20 minutes, and then discarded, with the filter. Then a normal oil should be used for 500-1000 miles, no synthetics or odd velocities. A good 10-30 or something close to that should be fine for almost anywhere(0-20W to 10-40Wish). 20-50W will function, but it should not be needed, that will create some wasteful drag. People successfully run 0-20W in newer engines.
The external chain tensioners have mechanical springs inside of them, those are their weak link. Those don't last 100k miles often, which is why you read of them failing so much. But even when new that spring isn't magic, it could become weak in a short time, not immediately of course. I would avoid saving or reusing any of those.
Those tensioners shouldn't allow a lot of movement of the cassettes, so if that ends up being the item that the noise takes you to, I don't think they will be the problem, unless the shop reused them etc.
Given a proper rebuild, the oiling system or initial break in wear make more sense to create slop in clearances, and noise comes with that. I'd be really sure that the external tensioners are new, and then look to the other stuff.