Any company operating under the provisions of sections 380.201 to 380.591 may invest in bonds of the United States, or of this state, or of any county, or of any municipality of this state, or in improvement bonds issued for street improvements in any city or town of this state, or in bonds issued by any county of this state for the improvement of highways, or in notes, bonds, debentures or other similar obligations issued by the federal land banks, federal intermediate credit banks, or banks for cooperatives or any other obligations issued pursuant to the provisions of an act of the Congress of the United States known as the Farm Credit Act of 1971, and acts amendatory thereto, or in bonds issued by building and loan funds, mutual investment fund, or in bonds or mortgages upon unencumbered real estate in this state or in any other state provided such real estate shall be worth at least twice the amount loaned thereon. The value of such real estate shall be determined by a valuation made under oath by two residents of the county where the real estate is located, and if buildings are considered as part of the value of such real estate, they shall be insured for the benefit of the mortgagee. To constitute a proper investment, county, city, town, school district, sanitary district or other political subdivision bonds or other evidence of indebtedness must be issued by authority of the law, and interest thereon must not be in default. Any company operating under sections 380.201 to 380.591 may also invest in certificates of deposit issued by any bank, savings and loan, or credit union, in an amount which is fully insured, and may loan or advance money to any other insurance company, provided that each such loan or advancement shall not exceed an amount equal to fifty percent of the average annual income of the borrowing company during the last five-year period. Any company may make such other investments as may be approved by the director.