By Kate Abnett BRUSSELS (Reuters) -The European Union was preparing sanctions on Russian oil on Monday, with possible exemptions for wary countries, and warned that complying in full with Moscow’s proposed scheme to receive gas payments in roubles would breach existing EU sanctions. The European Commission is expected to propose a sixth package of EU sanctions this week against Russia over its invasion of Ukraine, including a potential embargo on buying Russian oil – a measure that would deprive Moscow of a large revenue stream, but that has so far divided EU countries. Russia supplies 40% of …