Rick,
I use Mobil 1 and have been very happy with it. Here are some articles about long term testing.
Longevity test:
http://www.mr2.com/ARTICLE/Mobil1.html
Fleet test:
http://www.xs11.com/stories/croil96.htm
With a very expensive motor, I would rather be safe than sorry. The additional cost of
synthetic oil is minimal in the overall cost of owning a Cobra. I would be very cautious about the informal test mentioned by jschiller, while I respect the test effort, IMHO it has too many holes in the monitoring/testing procedure and an inadequate baseline.
Jim