Petroleum based Break-In Gear Oil allows gear teeth to break in quickly while improving their surface finish. By polishing the gear teeth, micro-pitting is eliminated to improve gear durability. A smooth gear surface can carry more load and last longer. The Break-In Gear Oil can be run for the normal break-in cycle, and then a synthetic racing gear oil may be used to get the most protection and efficiency from the rear gear and transmission. Viscosity typical of 80W-90.