I read an interesting article on engine oil a few years ago, when I got into a discussion with some people about changing intervals for oil on a car. I can't recall all the details. I had been changing oil on my vehicles every 3K since I was 16 years old, and had been using Castrol GTX since I owned my first VW, which was in 1969 (damn, I wish I had that Karmon Ghia now!!). The article/test took several (don't remember the exact number of test vehicles) cabs in New York City, built identical engine for each of them, used the same premium dino oil (don't remember the brand), etc. About as good as they could get with regards to controlling the variables in the test. Changed some of the cabs' oil every 3K, some every 5K, some every 7.5K, and another group didn't get changed until something like 15K. After several tens of thousand of miles on each vehicle, performing other required maintenance like air filters, "tune ups", etc., each engine was torn back down and measured for wear, signs of stress due to contaminants, etc. The findings showed NO appreciable difference between the engines with 3K intervals, and those with 7.5K. There was SOME wear on the 15K interval engines, but not dramatic. At that point, I figured you don't get much harsher driving environment than NYC cabs endure, so I compromised and started doing mine every 5k on my cars, back when I was still using the Castrol GTX. The Subaru regional rep told me that if I use Mobil 1 in my Outback, I was wasting product if I changed it before 7.5K. He said most people still stick to the old standards because it makes them "feel" better, not because the engine needs it. He said even 7.5K on synthetic was overdoing it, even with around town driving.
So, for me, it's not about the few dollars I save waiting to change until the motor has 5K (bike motor), but about not wasting the resource. That may sound corny, tree-hugger, Al Gorish, blah, blah, but I think I need to do my share to not dump perfectly good product.