The old adage "sell in May and go away" has probably caused a lot of investors to miss out on some promising stocks, and that's likely to be the case again this year. Instead of selling, May might just be an excellent time to buy -- especially for long-term investors looking for timely opportunities to add great companies to their portfolios. Three Motley Fool energy experts have already put in the work to help you get started, having identified integrated oil and gas super-major Royal Dutch Shell plc (ADR) (NYSE: RDS-A)(NYSE: RDS-B), surprisingly diverse sand supplier U.S.