For anyone interested in determining the optimal change interval for their type of oil and driving conditions, this company offers a free oil collection kit for laboratory analysis. (The analysis costs $22.50.)
http://www.blackstone-labs.com/index.html
They will give you suggestions for oil change intervals based on the laboratory analysis. This way you can avoid the unnecessary trouble and expense of too frequent changes. They will also tell you about such things as the presence of wear metals, silicon, coolant and other things that can let you know the condition of your engine and how it is wearing. Just thought I would share this information in case anyone else is interested.
Here is another site with interesting information: http://www.bobistheoilguy.com/whatisoilanalysis.htm