The normal boiling point of hexane is no hard limit. If oil contains 1% hexane, at +69°C the vapour pressure is about 1% of 1atm, so it doesn't boil. On the other hand, liquids evaporate already below their normal boiling point, just without boiling. Water dries off at room temperature too. And propane is stored liquid at room temperature, well above its normal boiling point, just under pressure.
What does the doc of the tankers say? I wouldn't be surprised if they limit the temperature range, for instance to avoid stresses due to thermal expansion.