International oil and gas stocks led the downside Monday as crude oil prices plunged to their lowest level in 5-1/2 years, hurt by weak demand and global oversupply. All 10 oil- and gas-related industry groups that IBD tracks slid more than 5% each. Brent crude fell 6% to $53.11 a barrel, its lowest close since May 2009. West Texas Intermediate crude settled at $50.04 a barrel after slipping 6% to $49.77 intraday.