This is the answer I got from Lake Speed at Driven wen I asked about adding break in additive to the BR40 oil:

"The BR40 does not need any extra additives, so I would not recommend using the additive with BR40. It can't help the BR40 work better. It already has the proper balance."

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......