LONDON, Oct 11 (Reuters) – Differentials for Nigerian crude oil grades continued to crash on Friday with the key grade being shown at its lowest level since 2005 owing to still rising freight rates as a result of U.S. sanctions on China’s shipping company COSCO. * Phillips 66 tentatively chartered a supertanker to ship U.S. crude from the U.S. Gulf Coast to South Korea for a record $14 million this week, sources said Thursday. * Traders and ship brokers said that rates