I have an 03 S with the JCW engine package. The car has 111k miles and uses no oil, it does leak a drop or 2. I have used Shell Rotella from new, changing at 5k miles. I think this is as good as you could hope for.

You N14 engine is known for carbon build up.  I've owned a 2007/2009 and now a 2011 (N18 engine).

My advice is to change the oil every 6 months or 6k miles.  Use the proper engine oil.  There is a particular rating you need to get that escapes me right now.  Honestly i found that the dealership sells the oil for a reasonable price ($5 or $6 per qt).  My dealer charges $69.99 to replace the 5.5 qts and filter.  Its done in less than hour and they server bagels and coffee.  

This is an area where the answer will depend on who you ask...  Most modern car manufacturers and oil companies do not recommend oil additives.  ZDDP levels is really not an issue for modern multi-valve engines, and can run the risk of damaging the cat with too high levels.

I recommend you simply stick to brand name full synthetic motor oils at 5w-30, or other weights recommended by MINI and change the oil more often than MINIs recommendations.

Ken

Just bought a 2007 mini cooper s with 65,000 and I was gonna get the oil changed but wondered if putting B G Oil additive hurt the engine.  The car runs smooth but would like to add a little extra to the engine oil to clean it up.  I also used B G 44K fuel system cleaner and I did notice a difference right away.