I am sure this one has been debated before, and I'm kind of curious about it but I was just thinking about the characteristics of two different oils. Would a thick 20 weight, say 9.1 cSt at 100 degrees, with a HT/HS of 2.7, that stays in grade offer significantly less protection than a 5W-30 with a viscosity of 9.7 cSt at 100 degrees and an HT/HS of 2.9 or 3. I know that most 5W-20s have a pretty beefy additive package if they are Ford/Honda approved, and I am wondering if a 9.1 cSt 5W-20 is really that much different than a 9.7 cSt 5W-30 if both hold their grade, which is the next question. I will have to read some UOAs. The application would be in a 1999 Malibu V6 with the 3100 low output (155 horsepower versus the later in model year 170 horsepower update) engine. Primarily lots of cold start, 15-20 minute runs, with the occasional long highway drive. It doesn't normally see heavy loads, extended high RPM running, or extremely high temperatures. I would imagine the oil temperature takes a significant amount of time to get to the point where it would be much thinner than the 5W-30. The "most severe" duty the engine would see, temperature wise for the oil, is probably a sustained RPM run at 80 mph with the air conditioning on full on an 85 degree day, and the only time the temperature really comes up is on the highway at higher speeds on a hot day, and sitting parked in city traffic. It very seldom sees enough runtime to get the oil temperatures up significantly in the stop-and-go situation. The change intervals are short, typically no longer than 3500 miles, so the oil would have life in it as it came out of the engine. The two oils in question are Castrol GTX 5W-20, and Havoline DS 5W-30. The 5W-30 is on the thin side but Chevron's claim is they start it at that viscosity rather than it shearing into that range. The HT/HS of most oils in the 30 weight conventional range are around 3.0, so I would estimate it to be 2.9 or 3.0 for that oil. The information is not published. The Castrol GTX is a pretty thick formula (as are all of their oils) and it is 9.1 cSt at 100 degrees, most oils in the 5W-20 grade are 8.0 (for Havoline) to 8.6 cSt, typically around 8.4 with a HT/HS of 2.6. If the HT/HS increases proportionally to the viscosity, I would say it is around 2.7. The funny thing is, I think the difference in wear would as much come from the poor cold flow performance of the GTX relative to the 5W-30 Havoline. I know Gary Allan has done different viscosity experiments, I am just curious what the range of opinion on this is from the AEHaas types to people running straight 30 weight.