Where is the tapping sound coming from, towards top of engine or towards bottom? When you used the thicker oil, did it slow it down at all? If it did, possible rod bearing knocking. If it didn't slow it down at all, means it is coming from the bottom/middle of the engine and is more than likely a main bearing. Better hope for rod bearing, main bearings a bitch to replace. Hope this helps. Just a suggestion, lol.