My less than scientific reasons: T6 makes the z idle smoother and the exhaust fumes have a different smell (smells like a "cleaner fume" than before - I have dp's and tp's). Engine seems smoother overall. I was also getting some smoke on deceleration and that has been reduced greatly. The motor revs nicely and oil pressure seems more consistent (based on lousy stock guage). I just don't agree with the large jug being only 4 quarts (vs. 5 for other oils).