The standard 0w30 and 5w30 oils are going to be in their sweet spot viscosity wise from 200-230 degrees. If you are regularly above that (i.e. dedicated track car or FI) then you might want to step it up to a 40 or 50. I can't see changing to a heavier weight oil on a street car as you are going to reduce performance of the oil in the 180-210 range, just makes more sense to install an oil cooler (the Mobil 1 0W40 is a possible exception). A 15W oil is not going to protect as well as a 0W or a 5W even on a 90F cold start. The winter weight isn't just for winter conditions, take a look at some viscosity charts. The quicker the oil gets to the ideal 10-12 cSt range as Nissan intended the better lubricated you are going to be.
Here's a good warm up chart. Note how thick the 15W40 is even at 35C (95F).
