I have started to get the dreaded low oil pressure warning so I am at a bit of a crossroads on what to do.
I am curious, what does this indicate (engine on its way out or something) and what causes it?
For some engines yes, for others no. If it caused by something serious like piston rings, worn bores or main/big end bearings then likely yes, and it's time for a new engine (unless you're interested in rebuilding it yourself). This usually happens on only extremely high mileage engines, or engines that have been seriously neglected.
Before replacing any more things on the engine, I would seriously recommend getting an oil pressure gauge plugged in. Had I known the correct oil pressure specs (which trying to find them online was like a finding needle in a haystack) I would have never replaced my big end/connecting rod bearings nor taken the oil pump out in my car, potentially saving me €€€. Here are the correct unofficial specs, since VW never offered a guideline for oil pressure PSI at idle (taken from a post on Audizine):
If it's something basic like the PCV, Oil level or filter then it's a cheap fix. The next step up in terms of cost would be a blocked Pick-up pipe (I forgot to mention this in my original reply). This can become clogged with random debris and/or oil gunk/sludge. You will need a few tools and Dirko/OEM VW sump sealant once you're done in there.
If inside the sump and the pick up isn't blocked, try and wiggle the end of the balance shaft at the opposite end from the sprockets to see if it is loose. If yes, replace the pump with an inspected used one, as a new pumps have ridiculous prices.
There is also a chance the balance shaft could have seized, but I think you would have low oil pressure all the time and hear a grinding noise from the engine if this were the case. This can be checked with the pump off, but by taking the pump off you are condemning your wallet as nearly every bolt holding the oil pump is a stupid VW stretch bolt.
The pump doesn't generate oil pressure per se, but rather the oil flow. This flow can be blocked by something (collapsed filter, junk in pick-up) or reduced from too much crank case pressure afaik (blocked PCV). The pressure is created by the tolerances in the engine bearings, a.k.a how worn they are.
For about the same price as replacing the pump, you can replace the big end/connecting rod bearings without taking the engine out, but this is another nightmare I don't want to get into. The chances you could mis align a bearing is quite high, and that would lead to catastrophe. This work was completely unnecessary on my engine, but by the time I found the correct unofficial oil pressure specs I had already taken the pump and rod caps off. I had taken off so many stretch bolts at this point it was worth just replacing the bearings while I was already in there.
I would have taken off the pump etc and done all this work had I known the correct oil pressure specs. This is why I cannot stress enough that you need to get a mechanical oil pressure gauge tester plugged in to yours Andy. It could have potentially saved me half a grand in Euros.
Once you find out your car is actually below spec, then it's time to decide whether to keep throwing money at it or not. Do keep in mind to get the true oil pressure you must drive the car hard for 20 minutes or so so the oil is fully warmed up. I did this in quite a hazardous way, where I floored it and my mate held his hand out the window to hold the pressure gauge with the gauge sticking out the side of the bonnet
. I wouldn't recommend this, but it did reveal the true psi.
When just letting the car idle for over 30 minutes, I would get 18 psi on idle. But after 20 minutes of giving it the beans it was at 16.5 psi. During the 10 minute journey home from where were testing it, it remained at 16.5 psi when at idle. In my head, I thought this was too low as the pressure switch turns on at 17.4-23.2 psi. Little did I know
the switch doesn't monitor pressure at idle, but at 1200-1500rpm If it stays it detects low oil pressure for over 1.5 seconds it will stay on, no matters the revs.
Of course this driving was done after replacing the pick up pipe, filter and PCV valve and with the correct Oil level (It was past max from the previous owner when I had the low oil pressure light). So I'd already fixed the oil pressure without realising it, but didn't know at the time leading me to replace the big end/connecting rod bearings