LONDON, March 30 : The European and African oil market is showing more signs of tightness with some crude differentials hitting record highs as peak summer demand nears and Asia seeks supplies to fill shortages caused by Iran's blocking of the Strait of Hormuz, now into its fifth week.  The Iran war has forced the shutdown of at least 10 million barrels per day of oil from the Middle East due to Iran's effective closure of the strait, and attacks on Iranian and other Middle East Gulf nations'