Kolibear,
I seem to remember reading somehwere (probably in an old flying mag) that if you do ground run the engine, you need to run it for quite some time (20 to 30 minutes) to make sure that the oil etc gets properly heated up and burns off any moisture and combustion residues such as nitrogen oxides. Failure to do so means that they can build up in the engine and cause all sorts of nasties to happen.
If I get a chance this weekend I may have a look and see if I can find it.
Brooklands