Robjv

Intetresting, appears typical "recorded info". After over 50 years experience the same question is unanswered. Political and environmental concerns out weigh reliability and long engine life. Auto maker will be happy if vehicles are crushed prior to 250,000 km. Engines in my part of the world experience harsh conditions, -40 to + 40 C so my info is based on this. Engines subject to severe use, have, and still are recommended to up the viscosity, even the newer high tech versions. Auto makers in efforts to comply with government emissions and other requirements quite simply just "water down" the oil viscosity. Also high stress engine in marine applications were recommended straight SAE 30, and 20 50. As for mileage, the old 5.3 engines of say, 1999 got as good mileage as the new engines and were emissions compliant. A lot of engineering, and over engineering has not made significant improvements. Reliability and long life are compromised, in fact, unfortunately no longer a consumer concern, Fact!