Lubrication is not a matter of viscosity but of film strength. Viscosity is of importance with oils in certain applications such as damping, where the drag imposed by a very thick (viscous) oil may be valuable - say the focussing helicoid on a lens. In applications such as a shutter or any fine, fast-moving mechanism, high viscosity imposes needless and harmful friction and can render some mechanisms inoperable. In cars, too - the days of using high viscosity oils in engines are long past. Modern low viscosity, multiweight oils with very high film strength give better lubrication, reach vital parts sooner from a cold start, and improve mileage - and modern engines are designed with them in mind. The oils suitable for a shutter have much less viscosity than sewing machine oil, they're almost watery.
Originally Posted by cliveh