[hunterk1]

I would say it's both.

How you drive and care for your car - peak revs, frequency of your trips to redline, oil & other fluid change frequency, etc, all clearly factor into longevity.

However, a bearing will wear more quickly if it is under a higher load. I would expect that most motors in stock configuration, being made of an iron alloy, do not have a fatigue limit - this means that theoretically they last for infinite cycles at the prescribed load. Increase the load (enough mods could do this) and you could come to a point at which the engine has a fatigue limit.

The question I would ask to the above statement would be, “How many mods would be too much?”, i.e. how many would you have to make to significantly impact the engine? Many of the common ones (CAI, exhaust, even Hondata) don’t significantly impact overall power (so they probably don’t cause problems on their own), but they do encourage the driver to rev higher, because the reward is greater.