I finally got the time to read your lengthy posts and found them quite enjoyable and informative. One of your summaries involved making the point that the most important thing was to change the oil. Whether it's at 7K or 3K, change it. I couldn't agree more. The only principal I've seen in this thread that I take issue with, was the implication that because the manual says 7K, this works for everyone's situation. And it can't hurt to change it more often than the book says.
Your oil research paper was great. There is only one area of it I would like to add to, only because it appeared that your research explained what the VI additive does, but there is more information that should be understood about VI additive.
First, the quantity of VI additive necessary to achieve a greater span(e.g., the span of 10W30, would be 20)is not a linear increase. I don't know mathematically what it actually is, but it takes a very large quantity of VI additive to make a 30 weight span. This makes oils such as 20W50 and 10W40 have an alarmingly large amount of this additive. The enormous amount can cause severe carbon problems, particularly in smaller, harder working engines.
If I remember correctly, there are polymers that are added to the oil which coil up when cold, making the oil thinner. When the oil temperature increases these polymers straighten making the oil thicker. I could be backwards on this, but you get the idea. These polymers can break down with heat and cycling which eventually cause the oil to lose it's viscosity.
What I read some time ago, indicated that a span of 25 was the maximum recommended for the smaller harder working engines, or for extended oil changes in higher temperature applications.
Thanks very much for your research, it obviously took much time, but is useful to all of us.
Have a great day,
[Edited by LarryBible on 01-09-2001 at 07:57 AM]