Engine oil viscosity is a measure of how runny it is, thus indicating its ability to coat and protect metal bearing surfaces from rubbing together metal-to-metal. Oil viscosity is quantified by a number, usually from 0 (very thin) to 90 (very thick). But this viscosity is inversely proportional to temperature, so hot oil is runnier and less able to protect big gaps as found in older engine designs, and cold oil is heavy and sluggish, draining some engine power and again reducing protection. So a fixed-grade oil has to be thick enough to protect the engine at maximum temperature but this means it will be very thick at cold temperatures, making the engine very hard to start. Cold climate drivers sometimes have to resort to heating the engine with a paraffin heater before they can be started!