Man, I just gotta weigh in on this one. Using synthetic oils may do many wonderful and almost magical things, EXCEPT lowering RPMs at any given speed (with all other factors being the same, such as using Drive vs. Overdrive). The engine has no choice but to operate the speed it has to, based on the gear selected, final drive ratio, and tire size. If you don't change those factors, the engine WILL be spinning the same speed at any given road speed, no matter what oil is in the engine, transmission, or anywhere else. There may be less internal friction (or maybe not, depending on the exact oil you choose), but it absolutely, positively WILL NOT have any affect whatsoever on engine speed.
Think of it this way, on a 10-speed (or however many speed) bicycle. Put the bike in say 5th gear, and pedal at 15 miles an hour. Stop the bike, and lube the chain with synthetic oil. Still in 5th gear, pedal at 15 miles an hour. The crank will have the exact same number of revolutions at that speed, regardless of friction. The size of the crank gear, the wheel gear, and tire size will dictate how many times the pedal has to go around to maintain any given road speed. The only way to change it is to put it in a different gear, or change the tire size. Period.
Did I just overdo that a little?