One reason to change oil is simply it is an anhydrous substance (darn been wanting to use that word since high school, course I'm not sure I spelled it right /forums/images/graemlins/blush.gif), meaning it draws water when in open atmosphere. That combined with if it has been used (engine ran) the by-products of combustion and the water create acid. That might reek havic on an engine /forums/images/graemlins/yikes.gif /forums/images/graemlins/hack.gif /forums/images/graemlins/grind.gif /forums/images/graemlins/weld.gif /forums/images/graemlins/eek.gif!!!
OTR trucks go 6-7k because of the miles they rack up in a small time, keeping the oil hot keeps water out.
Now for a total side note: railroad engines have the pH of the oil taken regularly because of the amount of acid that build up!