That's why we usually refer to them as idiot lights. Only really made for the general public when you loose total
oil pressure, the light will come on. If they made it to go on at say 20 psi, then hot
oil pressure on a worn engine would come on or flicker all the time. So they made it low enough that if the
oil pump broke, it would come on. Same with alternator warning lights. Unless you broke a fanbelt, a alternator can stop charging , but the light won't come on. That's why gauges are so much more important. But then most of the public wouldn't know what they were supposed to be reading.
JD