I think if you plotted viscosity vs temperature for the oils you'd see the curves intersect at some cold temp. You'd have to plot them to find that temp. Then decide whether your startups are usually above or below that temp. Note: kinematic viscosity is not linearly related to the cold cranking and cold pumping viscosities which determine the XW designation but Ohio isn't the arctic so I wouldn't worry about that.
from -40 F to +200 F, Pennzoil Plat. 5w-30 is thinner than GC at all temps.
The VI is almost identical, GC = 167 and Plat = 169.
GC is 12.1 @ 100C and Plat is 10.3
Those numbers can't be right - in order for an oil to qualify as a 5W oil, it has to be less than 6600cst@-30, and all of those 5W numbers aren't even close!
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Those numbers can't be right - in order for an oil to qualify as a 5W oil, it has to be less than 6600cst@-30, and all of those 5W numbers aren't even close!