Honestly, except for some confidential Intel documents (possibly) or talking to an engineer who helped design it, there is no telling what the max operating temp for a processor is. You'd need to have a few dozen system run for a long time before you can really start to crunch the numbers.
The temperature guides out there I take for what they are worth. There is no citing of sources or any tests done on large scale, but more of a common sense thing.
The "No telling what the max operating temp of a processor is" line is nonsense. There are certainly some things you can infer about max temperatures by merely observing what the processor does as it heats up with the correct software.
Intel CPUs will all automatically throttle back when they get too hot to operate safely. But how can you tell when this happens? If you're a programmer, it is simple. You simply look for Intel PROCHOT# thermal throttle activity bit. This bit flips from a 0 to a 1 if when the CPU is throttling down as a result of being too hot, and it goes back to 0 when it is running at full speed again. There is a second bit that flips from a 0 to a 1 when this throttling has happened since the last reboot - it acts as a "history."
Now then. The temperature at which throttling occurs is different for each model. For the E8400 it is 95 degrees C. That is, you just keep reading the temperature, keep reading the PROCHOT# bit, and then temp at which it flips from a 0 to a 1 is the max operating temperature, and for the E8400, it happens at 95. RealTemp, available from
http://www.techpowerup.com/realtemp/, is a program that can read the PROCHOT# bit and the temps.
The CPU throttles when the temperature is at an unsafe level. Therefore, if it is not throttling, it is operating at a safe level. Therefore, when PROCHOT# is 0, the CPU is running at a safe level. For the E8400, that max safe temp is therefore 94.9 degrees C.
BTW, "safe" is defined as "returns the correctly calculated result." The CPU lifespan may be decreased by running constantly at 94.9 degrees (in the case of the E8400), but Intel warranties the chip to operate at that temperature for at least a year. Programs like Prime95 can verify correct calculations well after the warranty expires.
That's good enough for me.
-Llama