Here's why it makes no sense that opening the cap hurt the oil lines. Oil lines are at almost no pressure at all times, hot or cold. Coolant pressure gets when hot and the main stress on oil lines is to prevent coolant from getting into the oil, not the other way around. Opening the cap released pressure and coolant boiled, which caused coolant to rocket through coolant passageways and hoses toward the reservoir. That wouldn't hurt a healthy radiator, but could be the last straw for a weak radiator. As for the coolant light, that can be caused by water in the electrical connector for the level sensor.