You can try to bleed the existing hydraulic lifters, per the FSM procedures, one by revving the engine in a certain pattern (slight chance of improvement), the other by removing them and hand-bleeding them (better chance of improvement), OR buy new 3mm orifice lifters off EBay and bleed those and replace the stock ones (best chance).
You've already done the best hands-off thing with the Rotella. That's good oil right there.
John B.