Hard to say without a bunch of UOA's, but my dad used to have a BMW 740iL, the dealer swore it would be fine to only change the oil when the service light came on. This was about every 15,000 miles on average. Dad kept the car until almost 100,000 miles, during that time we never had to add any oil and it never used any, still ran like a top.
(Everything else broke, but I digress...) BMW required synthetic and it held 12 quarts as I recall.