I think there is no direct relationship between flash point and oil viscosity. Take a look at the Mobil1 oil spec.

M1 5w-30 and 10w-30 has the same flash point, which is higher than 0w-30 and even 0w-40. So, there is no direct relationship between viscosity and flash point. It makes sense because how thick an oil is at certain temp should have little to do with how well it withstand heat.

Anyone know what's the flash point of a typical 5w-30 and 10w-30 dino oil? I assume they would be a lot lower than Mobil1. Of course, we can't tell how good an oil is just by looking at it's viscosity and flash point. However, my assumption is that M1 5w-30 provides better protection than dino 10w-30 even at 105+ temp, therefore, M1 5-w30 should be more than adeqaute for our engine all year long at various temp range (even 105+).

Yes, the manual calls for 10w-30 for 105+ temp, but it takes into account that someone will be using 79 cents a quart no name brand oil.

The bottom line is, I think both Mobil1 5w-30 and 10w-30 will be more than enough for our engine at high temp, but the better pumpabilty of the 5w-30 may lead to better protection during startup, when the engine needs it most.