You can test it for yourself however and leave a container of it sitting and watch how quickly it evaporates. Typically the higher the temperature, the more the substancewill evaporate. In any case, if you did use mineral oil to cool your computer,evaporation would be noticeable once the oil level drops.
Dear Divine Kebulu , The mineral oil generally can flash or evaporate at temperatures above 177°C. By apply vacuum could greatly facilitate the evaporation process. Since we know that pressure and temperature are directly proportional so mineral oil at high pressure will have high boiling point and at low pressure will have low boiling point. Therefore, when we maintain vacuum condition (pressure below atmospheric pressure) the mineral oil begins to evaporate at temperature lower than its usual evaporation temperature.