I see a lot of references in here to people running a different 'hot weight' oil in the summer vs the winter. Now I can understand that you would want different 'cold weight' numbers depending on ambient temperatures, so I can see using a 10W30 in the summer and a 0W30 in the winter in Chicago, for example. What I can't see is any effect of ambient temperature on the oil temperature when the engine is fully warm! My current vehicle runs a steady 210 degrees when hot, summer or winter. I suspect almost all modern vehicles do (except for air-cooled). What would possibly be the justification for running 30 weight at one time and 40 weight at another, AT THE SAME ENGINE TEMPERATURE?