OK, you are not hearing me....to scientifically test a theory, there is a control and variable. The control would be that both CPU's are identical (which is theoretically not possible due to imperfections in silicon and whatnot). The variable would be that one is OC'd and the other is not. OR one has a much better cooling solution than the other.
In the real world, it is not possible to get two identical processors (and I mean EXACT) so yes the manufacturing does come into play when overclocking, but as Sully said, a 10 degree difference in temperature will not make a noticeable difference in the lifespan of the processor itself unless we run the test and sit there and watch it for 30 years+. And manufacturing isn't the determining factor in the lifespan of a processor, just whether or not it can handle extreme overclocking while remaining stable.
I'm done with this argument. My head 'asplode again.