I had an engine shudder during my idle after startup yesterday morning. I checked my oil level and was a full X cross hatch height below the min oil line. I added 1.5 qts to get up to the max line. It made me curious how low does the oil have to go before the oil light comes on? Seems like I was 0.5 qts below min.
UPDATE: the engine shudder was ultimately due to a clogged PCV hose that got replaced.
The approximate answer for the cars that I have driven that take the standard 5-6 quarts is "about 2 quarts low". Maybe more, maybe less. Normally, you'll notice the light come on when taking a sharp corner as the oil sloshes to the side and the oil pump sucks air for a second.
But, the major point here is that the light means "low oil pressure", not just low level. That's important because low pressure means oil is not pumping through the engine properly, and serious damage isn't far behind after that. When the oil is too low to cover the pick-up tube from the oil pump, it sucks air, resulting in low pressure and "less than optimal" engine lubrication (to say the least).
If you see the light come on when taking a corner, find the nearest place to buy oil and add it. If you see the light come on when driving normally, pull over and turn off the engine as soon as safely possible. Don't drive the car until oil is added.
Typically, the oil light does not indicate a low oil level, but low oil pressure! Meaning that the anwser as to how low your oil level would have to be before the light turns on, would be "too low!".
It really depends on the car. My MINI Cooper doesn't have a low oil light, just a low oil pressure light. My engine was 3 quarts low and the light hadn't come on yet (and it only holds 4.5 quarts!).
