syntec is crap btw... its just oil laced with additives...additives are bad.
Besides oil wont really reduce your mileage noticeably. If you want better mileage, dont floor the car so much
Taken from the oilFAQ:
The wider the viscosity spread that you go for (Syntec 5W50 is the widest I know of), the more viscosity index improver (high molecular weight polymer) that has to go into the oil (and start with a lower viscosity basestock to get the 5W part). This polymer can be sheared by things like wet clutches, slowly losing viscosity. The polymer can also form valve deposits. Under normal operating conditions and maintenance, this should not be a problem. That said, engine failure due to poor oil (as opposed to LACK of oil or lack of required oil changes) is extremely rare with today's oils.