Did some interstate driving last night. Ambient temps 50F and 65 mph, hit and stayed on 190F with the cover on, about 20-30F warmer than with no cover.
Without the cover on extended downhill coasts (~3 miles) the oil would go to 150F, with the cover it only went to 160F.

Does your cover go on both sides of the cooler, or just the front?

I always use 5W30 Castrol Syn also, but that was when the temps were normally around 210F, where its the proper viscosity, but 5W30 at <180F is too thick and may not allow proper lubrication, is what I am worried about. And I think a 0W20/30 would be better for that temp.

We get fairly cold here in South Germany. The shops stick with 5w-30. You can go to a zero-# oil and Im sure the world will turn. I would not lose any sleep over 0 vs 5w.