Every 6-6,5000 miles with a filter change in the middle. With the quality of todays motor oil - especically synthetics as Mobil 1, keeping the oil clean
is a much greater concern than thermal breakdown imho. I think Mobil even says something like 10kmi+ change intervals are fine for light-service engines. Now, I'm not quite that brave yet
, but with the ammount of R&D that Mobil puts into testing, I'm sure they're right.
I think the taxi-test 400E
cited says alot. This is just an area where each individual's comfort level is the greater determining factor of oil-change intervals than is actual research findings/scientific study. And why shouldn't it be? If it makes you happy - your car probably won't complain either!
'77 300D 218kmi
Oh - and great idea intruder! Don't know why I never thought of that before - duh!