Vegetable oil in the water would not be needed or wanted
Is the heat pad plugged into a thermostat?
You cannot measure the temperature of a heat pad with a stick on the glass thermometer.
You need a probed thermometer so you know what the temperature is directly on the heat pad where the snake lays.
Soaking can be for numerous reasons, an up coming shed, too hot of a tank, stress from having a bad hide, mites, cooling off while growing follicles, and for just plain no reason at all.
The key is to go through the list of possibilities and nail it down.
Getting a thermometer that will read the heat pad temperature would be a great start.
*I have several breeding females eating voraciously(will bite for food) and wrapping water bowls and soaking, they are breeding though*