Definitely
synthetic, I prefer Mobil 1.
Quoated from a self proclaimed petroleum engineer, who worked for a motor
oil company and races vettes, from a corvette web site a while ago - "As far as viscosity, you should run the thinnest to maintain 10 lbs/1000 rpm, which is dependant upon bearing clearences. Lower viscosity means lower friction ( less heat build up, more hp to the wheels, and less bearing wear)." He highly recommended Mobil 1 above all other synthetics.