Originally Posted by wstar

It's not about the oil 'breaking down' at 260. We know a good synthetic will technically live through higher temps than that and still be usable oil. The primary issue is that viscosity drops as heat is added, and there's nothing Nissan's engineering or an oil company's engineering can do about that. If there was, they wouldn't still sell different weights of oil. They'd sell all 0w20 that was magically engineering to never lose viscosity to heat. There's a chart for the viscosity drop of various oil weights at various temperatures that's been posted in this thread multiple times I think.

The issue is that the 5w-30 you put in your car is getting too thin at 260+, which will cause wear issues in the long run. You can 'fix' this to a degree by running a much higher viscosity quality synthetic (such as 15w50), but you're trading cold-start protection when you do.