The first number in the oil designation, before the
"W", ie 5W-40, is the
"winter weight" of the oil and it directly correlates to the oils viscosity "cold" or at start-up. (the lower the "W", the easier an oil will pour or flow.)
The winter weight number has little to do with the viscosity of the oil when it is fully warmed up. The second number (-40) reflects the "operating temperature" viscosity which represents the oils ability to resist "shearing" when squeezed thru a small oirfice or bearing journal.
regardless of hot or cold climate, it is always an advantage to have a motor oil with a lower "W" weight because this oil will flow with greater ease to the vital parts at the top end of the engine. this characteristic is also crucial to start up lubrication in most if not all engine made today, as the tolerance of moving parts in the valvetrain and the like are very small and lack of oil will most definitely shorten their life. this is another reason why 5w type oil is recommended by the manufacturers. additionally if you live in the northeast, like me, you definitely notice the difference lower winter weight oil makes when it is cold out and the engine strains to turn over.
so if you live where the morning temps get down below 40 F, you may reconsider using that 20w or 15w weight oil. you may be damaging that expensive-to-repair motor at every cold start-up. just some food for though...
