Yeah...I believe oil for a car's engine is like a person drinking water to live.
Keep the oil healty and flush it out and refill regulary (every 3 months or 3000 miles).
Coolant is second....keep the coolant well maintained with BMW-only coolant (the blue-green stuff from the BMW dealership and NOT the gooeyneon-toxic green aftermarket crap)
Fuel/Gas is third...do NOT use 89 or lower octane on BMW cars. If you have a budget use 89 but don't ask alot from your engine if you do. Use 91 or 92 octane...
Oxygen/Air is fourth...make sure your air filter is clean and service regulary (twice a year) and your Air Flow Meter is working fine.
Keep your engine compartment clean and tidy...make sure your belts are in good condtition and your auxilary fan is working.
I'm sure there is alot more to getting more longevity out of your engine but these simple rules should keep you happy.
BTW most people end up getting new cars for other reasons than their engines dying....other complications can occur that shorthen the life of a car. like electrical failure, fuel system aging, interior upholstery decaying, orr simply new cars come out every year that makes us want to ditch our cars for them....
you guys seen the new E90s yet.....sweeeeet!