A high-temperature lubricating oil composition is provided, which is resistant to hardening and sludge formation, and displays minimal evaporation loss and superior thermal stability, even under practical high-temperature open system conditions such as those found in a tenter. A lubricating oil composition that provides excellent lubrication without damaging members used within working machinery is also provided. The lubricating oil composition comprises a polyolester based synthetic oil and a C12 to C72 fatty acid and/or a diphenylamine derivative containing an arylalkyl group with a number average molecular weight of 400 to 800.