There are lots of threads here where we discuss the benefits and pitfalls between group II, group III, and group IV based engine oils. Yesterday I came across a SAE paper (2001-01-1968) on diesel engine oils that concludes: “Extended oil drains in 1995 Caterpillar 3406 and 1996 Detroit Diesel Series 60 engines found no statistical difference in fuel economy or wear between a synthetic SAE 5w40 and an SAE 15w40 using A.P.I. Group II base stocks……” “Another study using 1999 Cummins N-14 engines found no statistical difference in fuel economy between a leading commercial synthetic SAE 5w40 and a leading SAE 15w40 using A.P.I. Group II base stocks plus a premium additive system…….” “Based on used oil analysis from the 1999 Cummins N-14 field tests, a leading commercial synthetic SAE 5w40 had statistically higher wear than a leading SAE 15w40 using A.P.I. Group II base stock plus a premium additive system. Both had the same A.P.I. CH-4-SJ/CES 20076/Mack EO-M Plus quality level.” The synthetic is a mix of 74.6% PAO, 0.4% Ester, and 25% Alkylated Naphthalene. It continues with 14 pages of graphs and details. A few days ago I would have told people I thought Synthetic was better, but maybe not worth the price in many cases. So could the difference be in our minds?