This document describes a methodology for establishing quantitative performance data for oil skimmers for recovery of oil with high viscosity, so the end user can objectively judge, compare and evaluate the design and performance of different skimmers. The methodology applies to testing in a basin and requires control of test oil properties and oil slick characteristics.The method is applicable to all types of skimmers provided that the equipment dimensions are within the physical limitations of the test basin. The test procedure provides full-scale test results for the unit tested, under controlled conditions, and for one or more classes of highly viscous