One important reason 0W oils were created was for hybrid engines. A hybrid engine operates through a constant cycle of starts and stops, depending on driving conditions, which leads to constant heating and then cooling of the engine. During these cycles, if the oil had to reach operating temperature to be at full fuel efficiency, only to cool back down when the engine cycles off, the optimum fuel-efficient viscosity could never be maintained and fuel economy would suffer. For this reason, car manufacturers needed an oil that was at optimal operating viscosity right at startup. That’s why many hybrids call for 0W oils in their specifications, although they also provide benefits for many other vehicles.
