Norway's $420 billion oil fund is rattling the cage of some of the foreign companies in which it has invested. As a shareholder it deserves praise for putting its head above the parapet. But as a sovereign wealth fund it is treading a fine line.

Norges Bank Investment Management (NBIM) has been stung into action by a combination of domestic political pressure to account for its investments and heavy losses on some parts of its extensive external investment portfolio.

NBIM has publicly chastised Volkswagen for its plans to take over Porsche assets as part of a cosy merger between the two German carmakers. A detailed letter to VW Chairman Ferdinand Piech, published in full on its website, doesn't mince words.

The fund's list of gripes is pretty long, ranging from conflicts of interests, a lack of transparency, questionable financial and strategic logic for the deal to concerns about the treatment of minority shareholders.

NBIM's intervention may well be too late to have any effect on the outcome of the planned tie-up. Only legal action by the fund, which had a 0.35 percent stake in VW at the end of 2008, could prevent the deal.