I used Quaker State and Penzoil without problems. I remember reading about Consumer Report best motor oils back in the early 90's. It ranked Penzoil as second to Redline(synthetic) and Mobil 1 (synthetic). I also read that Valvoline is the oldest oil company in the US from a Japanese car magazine. However, I remember from school. Wasn't Quaker State the oldest? I agree, that it is due to negligence of regular maintainence or something other than the quality of oil. I have a oil collection bucket(used for collection oil in oil changes)that got contaminated with coolant and water. It sludged really bad on the bottom. I couldn't understand why it was that way until, I remembered the cross-contamination previously with coolant and water. Note, the oil was not QS or Penzoil but, another brand. I feel another conspiracy theory brewing.