1999 Golf 4 - OIL LIGHT GOES ON AND OFF

The oil warning light is for oil pressure, and it means you're supposed to stop the engine immediately and coast to the side of the road. It's very possible nothing is wrong other than the sending unit, but I just answered a similar question for someone who destroyed their engine by continuing to drive for a couple of miles. The fact that your engine is still running is a hopeful sign the repair won't be expensive, but you don't want to take that chance. Your mechanic will test the actual oil pressure with a mechanical gauge, and if it really is okay, he's going to recommend a new sending unit.

That's not definitive enough to risk a misdiagnosis. There's two ways to approach this. You can replace the sending unit, and based on the recent history there's a pretty good chance that will solve the problem, but no professional wants to throw parts at a problem until he knows they're needed. The better way is to use a mechanical oil pressure gauge so you know exactly what the pressure is. The problem with that is when the light turns on so seldom, it's hard to tell if there's a problem unless you actually see the gauge drop.

A bad sending unit can act up any time and at any speed, but they tend to turn the warning light erroneously at lower engine speeds. Low oil pressure is also worse at lower speeds because the oil pump is running slower. The pressure could be just above the point at which the sending unit turns on. Lower engine speed and hotter oil that's thinner will both reduce pressure, possibly to the point the light turns on. If the pressure really is low, as long as it isn't 0 psi yet, the engine can usually be saved, but it won't last long like that.