</font><font color="blue" class="small">( I've often wondered if the tolerences in the motor may dictate the oil used, I would hate to have excess wear over time because the oil isnt where its suppose to be when it is suppose to be. )</font>
Yes tolerances (indirectly in terms of bearing pressures), rpm range, operating oil temp range - to name some key players - determine proper viscosity. Just as important, in terms of oil chemistry and additives, is the API service rating (that "donut" on the bottle), since only "C-" category oils are formulated appropriately for diesels, and nowadays typically CH-4 and CI-4. Doesn't get any better than CI-4 Plus, at least not until CJ-4 hits the shelves.
Sometimes there's specific tests and criteria beyond that to be satisfied (or so some manufacturers say to make you buy "their" oil).
Getting back to the weight, I usually take the tact of sticking with the manufacturer's recommendation (let's say 10W30) at the start. After I've racked up alot of miles (or hours) and things are worn in real good, I bump up the weight a notch (e.g., 15W40). Basic idea, try to compensate for bearing wear and bigger clearances with heavier weights to ideally maintain the same oil pressures as new. I'd be a little nervous about dumping a heavier weight in a new(-ish) engine, but if it's an engine where I'm changing the oil often enough, I often do it seasonally like some have said. That's just me.