I had a 74 Gran Fury HP 440 that exclusively used 10w40 Havoline the entire life of the car (late 74 to early '93) and it never had a drop of sludge in that engine. It developed massive leaks during the twilight of its years, but other than that the engine looked brand new on the inside. In fact, when I sold the car the guy that bought wanted to know when I had the engine rebuilt, as everything looked new when he popped the valve covers. I told him that the motor hadn't ever been touched internally and I think he thought I was lying. (After he rebuilt it he started using Pennzoil, which he said DID have a tendency to leave sludge. He went back to Havoline after 30k and never had another problem as far as I know.)