TEOSTO

TEOSTO is a family of standardized test methods developed in Finland to evaluate the oxidation stability of lubricants and the tendency to form varnish and deposits under high-temperature, oxidative conditions. The tests are used to compare oil formulations and to screen base oils for their long-term performance in engines and gear systems. TEOSTO methods are applied by lubricant manufacturers, research laboratories, and some quality-control facilities as part of product development and specification compliance.

In typical TEOSTO testing, a small amount of oil is heated in a controlled vessel at an elevated temperature in the presence of air and a catalyst, often copper, to accelerate oxidation. The test duration and exact conditions vary with the specific TEOSTO variant and the intended oil type. After exposure, results are interpreted by assessing varnish or deposit formation on copper surfaces or by analyzing changes in the oil, such as color changes or residue accumulation. The outcome is used to rank formulations in terms of oxidation resistance and varnish-forming tendency.

TEOSTO has several variants designed to address different oil categories and testing goals, including mineral-based lubricants and engine oils. While not a direct representation of real-world engine wear, the method provides a rapid, standardized means of comparing relative oxidation stability and varnish potential across products and batches. It is typically used in conjunction with other compatibility and performance tests to support lubricant formulation and quality assurance.