Low oil pressure 1.8 20V

Hi, my local garage is trying to find a fix for low oil pressure on my 1998 Audi Cabrio. It has got the 1.8 20V engine, but without the turbo that's
fitted to this engine in many parts of the world. It has done about 60 000 miles, and has been serviced regulary at a non-audi garage.
The problem only occurs when the engine is hot and only at idle.
I have read a lot about the "oil sludge" problem, but as far as I can understad this problem is related to insufficient cooling of the turbo. Can "oil sludging" also happen to my non-turbo engine?
When the mechanic inspected the oil filter, he found the bypass-valve had worked itself completely loose - it dropped to the floor when he removed the oil filter. The oil filter was changed about 8 months ago, so I'm guessing the engine has been without a bypass-valve for most of that time. I know very litle about engines, but I guess this really could be the source of the oil pressure problem? That is, the engine has used unfiltered oil for 8 months and the internals are now worn to a state where it can not maintain pressure. The mechanic noted that the oil looked very good (clean and without paricles), though.
On a brigther side, the oil pressure has only been messured by looking at the meter on the dash. The mechanic says he can't remove the cars sensor without breaking it, but I told him to breake it if he has to. The gass-meter has also been acted a bit strange lately. Are these two meters interconnected somehow, and woth investigating?
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First, the broken bypass valve is unlikely to have caused serious problems.
Second, the dash gauge is unlikely to be accurate.
Third, there is probably a port or place to attach a secondary oil gauge (and *accurate* mechanical one!) so you can determine the real oil pressure.
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And fourth, if you're using the oil pressure *light* as an indicator, forget it. The sensor/switch for that is notorious for failing. Before doing anything else (including paying for an accurate pressure check), replace it. It costs about US$8 and I'd bet you can't get a pressure check for less. -- C.R. Krieger (Been there; done that)
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No, there is also a meter for oil pressure in bars. It looked like it was alive and kicking - showing different pressure depending on revs. It turns out that it shows about one bar below the real pressure, so all I needed was a new sensor. If the car didn't have this meter, the garage would probably have checked the pressure first thing... I have forked out for a new oil-pump, but I'm happy to not have to buy a new engine :)
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