I removed the broken injector, just about to put in the new one. I notice something different compared to the other 5. The bottom of the dead one, where the nozzle is, it was fouled and had either slight varnish on it or oil. Either way it was dirtier than the others. Is this just the sign of a dead injector?

 

Nope - its merely a sign of a dirty injector.

 

Good - just wondering if this was a clue of oil going where it wasn't supposed to be - ie. some other pending engine problems.