HOUSTON, Aug 21 (Reuters) – Shares in U.S. oil producer Occidental Petroleum (OXY.N) fell to $56.17 on Tuesday, below a level that has routinely triggered purchases by its biggest holder, billionaire investor Warren Buffett’s Berkshire Hathaway (BRKa.N). Past multimillion-share purchases were so routinely timed to drops below $60 that Wall Street analysts called it “the … Read more