Your understanding is backwards. As oils get hotter, they thin. In what is termed a multiweight oil, the second number determines the thickness at operating temperature at 100c, the first number tells you the what cold weather pumping test an oil has passed. So for example, a 0w-40, 5w-40, 10w-40, 15w-40, and straight 40 weight oil will all have the same thickness at 100c of 12.5-16.3 cst. As the temperature decreases, the straight weight oil will thicken the fastest, the 0w-40 the slowest. The way that oils meet the cold temperature pumping test is a combination of the base oils used (synthetics are naturally multiweight to a degree) and pour point depressants.
Viscosities can be related horizontally only Viscosities based on 96 VI single grade oils. ISO are specified at 40°C AGMA are specified at 40°C SAE 75w, 80w, 85, 5w, & 10w specified at low temperature. Equivalent viscosities for 100° & 210°F are shown SAE 90 to 250 and 20 to 50 specified at...
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For your use of rolling gear oils, I would suggest looking at CSt at 40c which is closer to the operating temperature you are working with. As the oil will get thicker as temperature decreases, this is when you will have issues with the pawls sticking or skipping. If you stick with gear oil, I would consider something like Redline 75w140, which is an ester based gear oil and will have among the lowest change in thickness as temperatures change (but still substantial).