Mine has been absolutely troublefree. However, it is early!
I have had car for only two years - had about 100K when I got it - now has 120K. Get 25MPG, uses almost no oil between 3K changes on Mobil 1.
The previous owner, a neighbor and friend, cared for it quite well, but drove it very hard - 85 MPH open road - full throttle around town. Lots of oil changes.
Most of the information I have seen re: engine destruction says there will be little or no warning. There were only about 800 1991s and few thousand total 3.5L sixes in US. Not sure there is enough of a database to explain or project ...
How about yours?