Friction between surfaces is actually generated by zillions of tiny “welds” that form between the two. As sliding occurs, each weld forms, is stretched until it breaks, snaps back, and is remade farther along in a cyclic process that takes place at extremely high frequency. If the rubber is too immobile (extender oil viscosity too high), it cannot make, break, and remake enough bonds rapidly enough to generate maximum friction. This is why rubber lacks grip on lap one. Increasing the rubber’s mobility by use of less viscous extender oil moves the point of peak performance downward in temperature.