You need to “google” nikaseal and aluseal engines. BMW had tremendous problems with the engine blocks right around these years (notice I say right around because I can’t remember exaxtly where th bad started and the good started).
The cylinder blocks on these engines would wear prematurely causing loss of compression and oil consumption. BMW stated the cause was “low sulphur fuel” (I always thought that was a diesel thing) but in any case many engine blocks were exchanged. I really can’t believe there are cars out there that have the problem and not been fixed,but maybe.
This engine debacle was propably the largest,greatest problem in the history of BMW.
Manual trans 540’s were very hard on clutches or should I say the owners of manual trans 540’s were very hard on their clutches,driving a manual trans 540 hard is one of lifes little pleasures.
If you have the money my choice is for the 540,and its a easy one.Very high expensive maintiance if you want to keep the car in top form.