I didn't read through every post on this thread. Oil topics get hot and crazy. The only real way to tel is to perform an oil analysis and use a good method to do it. I am a lubricant engineer and work with many automotive companies, including GM, Mercedes Benz, Porsche, Nissan, Kia and Ford to anme a few. You need to establish a baseline to really determine when to change your oil.
I recommend choosing the oil you think is the one you want to use. Mobil 1, SynPwr etc. At 3,000 miles on the drain pull out a sample and have it tested. It will come back fine and recommended to continue use. It will be flagged if there's anything abnormal. Next run out to 5,000 miles on the same oil. Compare it to the 3,000 sample. It will more than likely still be good for further use. Run again at 8,000 miles and repeat. You will be able to see when the oil is ready to be changed. The scary part is that some oils just hit a way and go from being reasonably good spec'd and drop off fast.
In the analysis you will see contaminants, such as soot, silicates (dirt), wear metals if any, water, fuel dilution. The company that provides the service can help you read it. Once you have fugured this out you will not have to do this every oil change. I recommend doing an oil analysis every couple of oil changes (only when you drain) not several times throughout the change. You will be looking for any abnormalities that can lead to a reduced engine life.
Diesel guys. If your doing this and don't have a fuel dilution problem the best indicator is to change the oil when the TBN and TAN numbers are close to the same number. TBN starts out typically from the 9.5 - 13 range and TAN an 0 (zero)
Here's a very trustworthy company the we use. You can mail in your sample from anywhere in the country.
Petroleum Technologies Group L.L.C.