Some mechanics are basically hack mechanics. The thing breaks and they just move on = not good!
To illustrate this fact, I redraw the Oil Filter Cap.
The 2 tiny O-rings' sole purpose: when you change the Engine Oil, the Oil is drained down the Oil Pan to be drained out and disposed of.
When the engine is running, these 2 tiny O-rings prevent oil from being pumped back to the Oil Pan. So when this tip (with the 2 tiny O-rings) is missing, some oil is pumped into the Oil Pan, thus "low oil pressure" light on dash during idle rpm
At higher rpm, there is more oil pressure, enough to extinguish the light in the dash, but might not be enough for proper engine lubrication
PS: This is hypothetical but I will try to be as close to the numbers as possible.
- Usually the oil pressure light goes out when the pressure is above 8psi or so.
- OK, your Oil Cap Tip is gone, when you rev the engine to 2500 rpm, oil pressure is let's say 15 psi (instead of 30 psi), enough to turn the dash light off.
But the engine is designed such that at 2500 rpm, it needs around 30 psi for proper lubrication.
So at 15 psi, you risk engine damage.