I purchased a '93 520i about 3 months ago, and since the oil was quite dirty, I changed it with the filter even thought the service indicator still showed plenty of time left (the previous owner had the vehicle regularly serviced at a local dealership). Now, 700 miles later, the oil is getting dirty much faster than I expected. Since I always change my oil religiously at or before 3000 miles, I decided to use dino oil. In this case, I used Castrol High milage 10W-40 as recommended in the owners manual for winter conditions.

Being new to BMW ownership, I guess my question is: Is this unusual for an older BMW? If it continues to degrade at this rate, I'm not sure I feel comfortable even letting it go to 3000 mi.

What would cause it to blacken so quickly? Detergents in the oil? Deposits in the engine? Any other causes?
Thanks, JT