No way!

Dont use 15W50 unless your engine gets really hot!

and it does not!

For a knock, I would fix it as fast as I can, forget the dealer, or just sell it!

For oil use (Castrol GTX high mileage SAE 10W-40), which will give you a thicker oil than 10W-30 but not to thick as the 50!

but I would take it to some where else than the place you took it and do not tell them what the honda dealership said and let them figure it out there selves!

But all the info I gave may not be the best, just go with what you think you should do!