[M.D. Holloway]

Increased fuel eco is the first thing many folks notice when going to a blend or straight synthetic - this is real noticable with older engines. There are a few reasons why; increased lubricity, stable viscosity means better film formation, many blends and straight synthetica actually help remove deposits that have formed. This is especially true of the esters and to some extent the PAO's.

Better milage can also be achieved when you switch to a lower viscosity oil - say a 10W30 as opposed to a 15W40. Be carefull with that though because some engines get kinda rattled when you introduce a thinner oil to the frey - it may reduce resistane but it can also mess up the main bearing lubrication resulting in increased wear .