st4
Banned
Only on certain M-series engines.
I've never heard of mechanical failure on the BMW forums for IL6s.
The seals on the other hand do need renewal.
The procedure section of this website shows how to do it
http://www.beisansystems.com/
They can hydro lock the I6's and are prone to this. Its a good idea to check the oil regularly on them, and check for a milky residue. Its usually avoided by regular use to be fair but a fair few 3 litre X's has this trouble.
The newer valvetronic engines (4 pots and some 4.4i V8's-the 330bhp ones not the weedy 286bhp ones) seem fairly strong and trouble free. The valvetronic cars are more powerful and more economical too.
if MPG is such an issue for the OP-there is always a nice reliable torquey CDi