This excerpt taken from the PBR 20-F filed Jun 30, 2005.
Oil products prices
The prices for fuel oil did not grow as much as other oil products. With the increase in demand for oil products, refineries used more heavy oil that produces more residues, including fuel oil, than light oil. Because the demand was concentrated on light and medium oil products, there was an excess supply of fuel oil. This generated an increase in the price difference between heavy and light petroleum.