Most of the ppl replying to this post are speculative about whats happening with their oil. You can't know whats happening with your additives and contaminants unless you do a proper oil analysis. Whether a mechanic says it 'looks dirty' and whether the oil is analytically dirty (soot content, particle content, chemicals, TBN, etc) are two different things. For one - mechanics want to make money by changing your oil. Two - most mechanics are trained in the older school of thought, and are resistant to accepting the capabilities of the newer oils.Most likely despite your driving conditions -short trip or long trip-, engine to full temp or not, you're dumping out oil thats still fine to use. There is definitely something to be said about changing the oil filters midway through your drain interval, more so if you take lots of short trips, drive hard, use crappy fuel, etc. Do yourself a favor, spend the $14 bux, send off a few ounces of your oil in their ready made (no spilling) test kits, and find out for yourself exactly how your engine is running. Not only will it tell you how clean your oil is and how long it can go for, it will show indicators of many other conditions. For example - if a certain wear metal content or chemical content is high, you would be tipped off to bearing wear, or an air leak, etc. Its great for preventative maintenance and diagnosing probs before they wreck your whole engine. Aside from that it saves you money by letting you know your exact ideal oil change intervals. Below is a link to the site - kits can be purchased online thru the amsoil website - at least that's where I bought mine....good luck guys!
http://www.oaitesting.com/