I know I am a bit late with this reply. My experience has been with the 6 blade fans and I have not suffered over heating which leads on to having a good radiator. Going back through service records fromthe factory quite a lot of cars came in to have the 4 blade fans replaced for 6.
Oil filters were an optional extra and they are of the bypass type. As long as you use a non detergent oil which take out all sorts of bits the engine should be ok. My M has not got a filter and has been fine for the 15000 miles.