If you have not seen it, go here...
http://www.trustmymechanic.com/motor-oil-bible.pdf
Although the guy seems very pro-synthetic, he does have some valid points and a great speadsheet on the performance of various oils at different viscosities. A decent read.

 

Had this thing for years now. The guy knows heaps about oil. Best thing you can do is buy synthetic and send a sample to Blackstone labs, to figure out what kind of protection instead old wives tales (I change my oil at 2k, 3k, etc). Change the oil when it needs to be changed instead of some 30 year old interval