I recently bought a 66 Coupe w/289/2V.  I noticed the oil was kinda dark.  I changed it and put in 5W-20W.  The engine began to peck (valves, I think) and the oil pressure was low.  I changed that oil and put in 10W-30. It still pecks, although the pressure is almost normal.  Do I need to change to a higher viscosity or is it possible the guy I bought the car from had put in some sort of additive to quiet the car?
The car runs great and doesn't smoke and has plenty of power.  I use 89 octane.  Any advice wouold be appreciated.  EOD8403