Intended to finance Norway's welfare state when the country's oil runs dry, 65.9 percent of the fund is currently invested in equities, 31.6 percent in bonds and 2.5 percent in real estate.
The Norwegian central bank, in charge of managing the fund, is always on the look-out for ways of diversifying its investments and maximising its profits.
In the letter to the finance ministry, the bank recommended investing in unlisted groups, citing historical data which suggested they could offer a "slightly higher" return than listed companies.
The number of listed companies appears to have declined in recent decades, the bank argued, pointing to a nearly 50-percent drop in the number of American companies listed in the United States since 1996.
In practise, the bank recommended starting with investing in or alongside private equity funds.
It is now up to the finance ministry to review the recommendation before submitting a proposal to the parliament, which has the final say.