The only need for additives I would see are primarily for lubricity, and secondarily for cetane boost. Lower lubricity will have a cumulative detrimental effect on the interior working parts of the fuel injection system, while lower cetane primarily results in reduced combustion efficiency and cleanliness.
The biggest problem with diesel fuel in North America is reduced lubricity compared to European and other regions' diesel products. American diesel is known to have the worst lubricity (by national specification) at 520µm wear scar rating. 'Wear scar' refers to the size of the pattern of wear between two metal parts in a standard test jig; a larger wear scar on the test metal is worse than a smaller wear pattern. American diesel fuel specification (ASTM D975) requires lubricity that results in a scar pattern on the test metal to be no larger than 520µm. Canadian specifications require a maximum of 460µm. Most European countries have wear scar ratings between 280-320µm.
When considering what kind of additive to use, Arlen Spicer's lubricity report published in 2007 is pretty much the "state of the union" regarding qualitative and quantitative assessment of diesel additives available to the North American market.
Diesel Fuel Lubricity Additives - Study Results
by The Diesel Place and A.D. Spicer (2007)
In the end, the best lubricity additive was in fact addition of 2% soy biodiesel, followed by OptiLube products, Stanadyne Lubricity formula, Power Service - Diesel Kleen with Cetane Boost, and then some products that actually made lubricity worse!
Since up to 5% bio is okay by BMW, the best lubricity "additive" would be the 2% bio option, but if I did use an additive, it would be something like OptiLube Summer Blend or Stanadyne Lubricity Formula (protection and economical).
My 2¢
Cheers
D.