I don't know if this was ever posted here, but I found a nice (but long) paper from EPA going over amounts of CO2 (and thus fuel) saving from various technological advances in modern cars. It's interesting to see that low friction lubricants contribute only 0.5% to emission/fuel savings according to EPA. One of the greatest savings are from simple diesel engines, only slightly surpassed by hybrids. This is on page 6, table 2.2-1.
http://www.epa.gov/OMS/climate/420r08008.pdf
http://www.epa.gov/OMS/climate/420r08008.pdf