Get a Bluetooth OBD2 reader and forscan. Scan for codes and read up on monitoring PIDs. Most likely you need injectors. Possibly a FICM. You can start by replacing the oil and fuel filters with OEM. If you're lucky, you won't have blown head gaskets. This is going to be expensive. Don't just throw parts at it and don't take it to a shop that isn't a reputable 6.0 shop.