OEL Dashboard Light: What Does It Mean?

Hello,


Here's something I observed with a friend:

He drives a Golf 3 TDi (90 horsepower).

When starting the engine or turning the ignition key, the 'OEL' light flashes a few times in the area where the total mileage is usually displayed (on the speedometer, at the bottom).

I didn't even know that something like that exists?
Engine oil level is okay. Which oil level is being referred to?
What does that mean?

Regards,
Daniel.

Everything is in the operating manual , and the car would like an oil change .