Excellent posts by all. With all due respect, very little science though.
Who is getting oil analysis done on their used motor oil? I have. Results (5K between oil changes at the time, Mobil 15W50, converted at 60K) were excellent - no indication of wear, or abnormal particulates (was curious about K&N filter, I've since removed it anyway). This is my wife's primary car; can't get much more severe service than that!
A comparison of dino vs. synth vs. intervals, using oil analysis, would really help with this discussion and perhaps end it once and for all.
Ever wonder why the oil companies haven't done this (or released the results): "Based on our testing results, we find longer drain intervals to offer owners no substantial decrease in the performance or longevity of their automobile engines. Please, buy less of our oil by extending your drain intervals". Yeah right.
89 300TE 165K
84 733i 135K
Mobil 1 for both, twice per year
(well, they both use a little oil, so there is *some* fresh oil added between changes)