Low oil pressure warning lights and charging system warning lights on the instrument panel were relatively simple sensor circuits that would turn on a light to alert the driver to a problem. (yourmechanic.com)
Overall wear on the engine or the oil pump getting worn is resulting in low oil pressure. (mechanics.stackexchange.com)
I bet it's just too thin an oil and running the car on hot days lowers oil pressure enough for the warning light to come on. (mechanics.stackexchange.com)