Our 2003 Park Ave just turned 155,000 miles. Bought seven years ago at 30,000 miles, and the only non-maintenance items so far are one tie-rod, an egr valve, and climate display (easy fix with a soldering gun).
Not the most exciting thing to drive, but my wife and I love the comfort and reliability. After looking for a year, I found a 2005 Park Ave with only 14,000 miles, five hours drive from us. Brought it home the next day, and now it's our garage queen until our 2003 dies (who know when?). I kick tires on new or newer cars occasionally on the dealer lots, but when I get back in the old Buick I feel right at home. So, old Buicks forever?
Mobil 1 5w-30 6000 mile OCIs, usually about 40% on the OLM.