"Group V oils are by regulation categorically separate from other oils based on their specific gravity. Any oil that has a specific gravity higher than 1.0 or API (American Petroleum Institute) gravity value of less than 10 is a Group V oil. This classification is based on concerns that these oils have characteristics and behaviors that may be very different from other typical lighter oils. Some of these differences may lead to non-floating behavior of these heavier oils or fractions of these oils thereby presenting very different concerns with regard to environmental impacts and response capabilities."
USCG Sector Mobile Digital Area Contingency Plan: http://ocean.floridamarine.org/acp/mobacp/