Was surprised to hear about the yellow bottle Penzoil. Back in 1970 I bought a 69 GTO all OEM and ran Penzoil 10/40. Ran that car to 92,000 miles and pulled the motor as I broke a ring. Decided to do a complete overhaul since I had it out. Never had any sludge anywhere. I babied that car and maybe that had something to do with it. Plus I did frequent oil and filter changes at the DX service station I worked at.

 

Back in the 1980's and early 1990's I used nothing but Penzoil and never had a sludge issue - I was told by many to change my brand as it created sludge. In '94 I bought an '84 Olds Delta 88 from the original owner with a thrown rod, when I took that engine apart to remove it there was so much sludge/wax buildup that the turkey pan intake gasket left an impression! The original owner used nothing but Quaker State. I think it was not so much the oil as the maintenance schedule and driving conditions.

I change my oil often back in the 1980's because I drove less than 2 miles to work each day. In the '90's I drove 21 miles one way and extended my change interval and had no issues. In the mid '90's I started using AmsOil and never looked back.