In my 1990 E34 535i, 5 speed, 103K miles, I use dino oil. I found that
at about 2500-3000 miles, my oil is black and very dirty. So it gets
changed. I think more important than mileage is the type of driving you
do. If you drive mostly freeway with very few stops, then a longer
change interval is warranted. However, if you're like me and live in a
City with lots of hills and stop and go driving then a shorter change
interval is needed.
Btw, I tried synthetic oil once in my car and it burned, fast. At about
1000 miles, I was down almost 2 qts! I added oil and still it burned.
At about 2500 miles I changed it back to dino oil and its runs like