Benchmark U.S. crude for June delivery was up 51 cents at $100.76 a barrel at 0800 GMT in electronic trading on the New York Mercantile Exchange. The contract fell 51 cents Thursday to close at $100.26 a barrel. Brent crude, a benchmark for international varieties of oil, gained 76 cents to $108.80. Pro-Russian insurgents in eastern Ukraine are planning a referendum on independence over the weekend, in apparent defiance of a call by Russian President Vladimir Putin to put off the vote. Traders worry Russian crude exports could be interrupted if further instability in Ukraine results in stronger Western sanctions against Russia. – Heating oil added 2 cents to $2.94 a gallon