Actually, I too have the M30 "Big Six" engine in my 1990 E34 535i, 5
spd, 102K miles, and at least for the E34, there is "one weakness" that
isn't found in earlier versions like the E28 and E24. That is the E34
535i are known for its blown headgaskets. Earlier this year, my car
blew its headgasket at 97K, and I was religious about changing my
coolant with only BMW blue stuff every 2 years. If you go to the
various E34 forums, you will see this is pretty common.
In fact Bruno's excellent E34 website has a complete write up on
repairing blown headgaskets:
Nevertheless, the repair cost me $1100 and the car now runs good. Its
strong and I expect it to easily last 250K or more.