Singapore’s four oil retailers have raised gasoline and diesel prices to their highest levels.At S$2.147 a litre before discount, Shell’s premium grade is now the most expensive gasoline in Singapore. (US$1=S$1.46).The major, together with Singapore Petroleum Co and ExxonMobil, followed Caltex which had raised pump prices by five cents a litre on October 23. Caltex’s…