300E valve noise at start-up...

[Bill '90 300E]

did the search...didn't find much...'90 300E, 147,500 mi., very well maintained since new...recently changed to Castrol 20W-50 (I know, not recommended oil) and have noticed valve noise (tapping) at start-up....it goes away w/in 3-5 seconds (after oil pressure comes up?)...I think I may know the answer before asking, but I've also read that a 3.0L may need valve work around 150K miles...opinions? experience? as always, thanks in advance for your help, Bill