Every time I am driving a long distance at 120k/.

Oil is thicker when it is cold. When your engine is warm (after driving a long time) the oil is thinner and your
oil pump has a harder time to keep pressure up.

You might also have worn bearings in the engine that is causing loss of pressure due to excessive clearance
between bearings & shafts (crankshaft, connecting rod journals), that thicker oil can handle, but warm thin oil
can not.

You will need to have the engine inspected for wear, and possibly start with a new oil pump.

Thomas

If engine is heavily sludged, the oil strainer in oil pan could be partially clogged resulting in insufficient oil getting to oil pump.

Yes of course, sludge!

Although not that common on the 1.8L & 2.0L A3's (from what I have seen). It can happen.
Easy enough to check by removing the oil pan and having a look at the pick-up screen.

Image of clogged screen attached. 1.8T, hard to see the sludge with the glare from the
flash, but you can see how black the rest of the pump body is to the right.

Thomas

Mokgadi,

Thank you for the input but I don't think the problem is the same.

Logan's problem has the lifters creating noises which means oil pressure has dropped. An oil switch fault should not cause that, it would flicker due to inaccurate reading only but not affect the valve lifters.