Swiss trading house Mercuria has been awarded a tender to buy up to 2.8 million tonnes of straight-run fuel oil (M100) from Rosneft at a slightly lower premium than the Russian firm's last sale of the grade, trade sources said. Mercuria bought the Russian heavy distillate at a premium of around $86 a tonne above Singapore's 180-centistoke spot quotes, about 2 percent lower than Rosneft's last sale, the sources said on Tuesday.