Normal wear and tear.
In the twenty year's I've owned it, it has never let me down. It always starts, even at minus 40 without being plugged in.
It's not the fastest car, but still easily runs at over 185km/hr (115mph), and is rock solid on the road.
The most amazing thing - 860,000kms (approx 535,000 miles) - and with very little maintenance. In the life of the car I have replace the front brakes three times, the rear brakes twice (although it's just about time to do them again). It has worn out only one alternator, fuel pump, water pump and exhaust in it's life! Other than changing the oil religiously, and the timing belt every 100,000kms - it hasn't required much maintenance.
I have never heard of any other gas powered car come close to this kind of mileage! Treat one right - and it will repay you 100 times over!
How much did you pay for your 528 when you bought it new in 1983?
My 1983 528e cost $24,000 from Evergreen BMW, Frederick, MD. It, too, is still running and is a head turner.
535,000 miles!?!?! Oh my God! That's the most I've ever heard of. You should write BMW NA or Roundel/BMW CCA if you have the documentation to prove the mileage.