Try switching to a higher quality oil like Valvoline or Castrole, Penzoil is not the best oil and doesn't have the viscosity needed with bigger oil clearances. Also don't use synthetic, it runs thinner than conventional oil.
As mentioned by others above. I would go with a 20w 50 or 15w 40 especially if you have upwards of .003 oil clearance, that is too much clearance for standard street driving.
If after switching to a thicker higher quality oil the problem doesn't go away then you will need to consider pulling the engine and doing a thorough inspection. You may have some serious clearance issues or the oil pump may be defective.
