Try a 5W40 or 10W40. Have you cleaned your injectors? Stuck injections may be diluting your oil.
Agreed. Any of the GM V8's I have owned in the past were notorious for various injector issues including fuel dumping.
Also may wish to try Pennzoil Platinum Euro 5W40. Not the thickest of 40 weight, but exceptional at reducing valve train noise and provides superior protection based on my experience with it in light-duty diesel engines. Has even helped with hydraulic lifter tick in VW VE diesels (my own experience). If that doesn't solve it then throw some Rotella T6 or Valvoline Extreme Blue in 5W40 flavours in and see if that helps.