New to me car. the main issue is the low oil pressure messages and CEL come up after anything close to spirited driving. I have been reading up on it is likely sludge built up in the engine. I saw a youtube video where the guy had the messages and CEL, and he took the oil filter housing off and it had a lot of sludge. He cleaned that all out and said the issue messages stopped. I can easily enough start there, but I cant understand how even if there were sludge in the bottom of the filter housing, how that could be enough to block the oil causing the CEL and messages? Has anyone had the oil pressure messages and figured out a way to cure it?

So now that things are quiet around here(I remember when there were half a dozen responses to almost every question within a day or 2), I thought I would share some more info to anyone who may come across this thread in the future, and try to save them some time. I have learned that the 2 most common causes of the low oil pressure message on the S60(and the same 5 cyl in the XC90, V70, 850) are either the screen on the oil sump pickup is clogged with bits of carbon, sludge, or debris, and the other main cause is the O rings that create a seal between the oil pan and the engine block failing and letting air into the oil. To check for the failed O rings pull the dipstick after the car has been run at highway speeds for at least 20 mins or so, and look closely to see if there are tiny bubbles in the oil. That is caused by air getting in through the failed O rings. If it is a clogged pickup screen, there really is no way to tell unless you have a scope camera and feed it in through the oil pan drain plug hole. In either case you have to take off the oil pan, and both the O rings and oil pickup should be inspected to determine the cause, and the O rings should certainly be replaced regardless any time the oil pan is off. There is also an O ring to seal the oil pickup tube.
There is a 3rd possibility, that might be the easiest place to look at first, and that is the connection of the wire to the oil pressure sensor. But I suspect that would be more of a random warning message, and not like the way I experienced it.

In my case, we just bought a 2006 S60 with 180kmi on it. It was turned into a dealer by the original owner and went to auction, and the shady guy who bought it there sold it to us a great car without issues. The original owner surely followed the book and changed the oil at 8000 mile intervals(or more), and that is just crazy, especially on a turbo car. So it is caked up with burnt/carbonized oil and sludge. Then on day one after driving it a bit spiritedly, when we came to a red light, the no oil pressure turn car off immediately message came on. So we did that, waited a few mins, restarted the car and the light was gone and all seemed fine. Then again a couple days later a low oil pressure message showed up. If I was setup to pull the oil pan I would, but I cant do that. So we drained the oil, and I looked in the catch pan after pouring out the old oil. There was a small piece of rubber maybe 1/4", that I assume is a piece of a ring or seal, but no metal bits which is a good thing. If you have metal bits that glisten in the light, then your motor is in even worse shape(think bearings failing due to no oil-bad). But the piece of rubber was not a good thing to have in the oil, as that alone could block a section the oil pickup screen. What happens is the screen gets clogged and then it actually sucks up into the pickup tube, which then seriously limits the surface area that it originally had, and now if some bits of carbon or debris get picked up, you have the low or no oil pressure issue. I will take it in to the shop and have the pan dropped. I think the 'book' shows 3.5 hours labor, which of course at an actual Volvo dealer is probably something crazy like $2000(and they then want $500 per O-ring!), so I will try the independent mechanic I know to do the work. I hope this helps anyone who may have the same issue. Lesson learned is always test drive any car you buy by driving it hard, and then assessing and lights or noises that are revealed.

Update-
I was going to have the pan dropped to investigate further, which would have been a good idea, however I was pressed for time as my son was going to take this car on a long journey, and was leaving before I could get the car scheduled and into a shop for the work. So I thought through the issues and came up with this method. I am NOT saying that you should do this, it is just what worked for me. I came to believe that it was the screen on the oil pickup was clogged, and came up with a way to 'clean' it without removing the oil pan.

We drained the oil while it was hot, and took out the oil filter. We cleaned all the crud out of the filter housing with carb cleaner spray and put it back on with no filter in it. Then let the car cool for an hour, and then added 4 1/2 quarts of kerosene to into the oil filler. We pulled the fuel fuses out so the car would not start. Then we just tapped the key enough to get the motor to crank a little bit. I did not want to crank it over fully and distribute the kerosene all the way through the engine, rather I was thinking by pulsing the kerosene up and down through the oil pickup tube, it would hopefully dissolve and knock out the crap that the was blocking the screen. We did about 50 cycles like this, until the battery go weak. A couple of times the motor did crank a full revolution, but we tried not to. Then we drained the kerosene, and took the filter housing back off. I expected the filter housing to be full of kerosene, but to my surprise it was empty, except for a glob of black thick sludge. Maybe 2 tablespoons worth. I still have no idea where this came from, and how it got into the housing, but no kerosene did. We put a fresh filter in, and refilled with fresh oil. Started it back up, and then drove it for about 500 miles. The oil pressure light did not return at all during this time, but the car was not pushed as we knew there would still be some amount of residual kerosene thinning the oil. Then we drained it and filled with mobil 1. The oil housing had again accumlated a fairly large amount of crud, as did the filter. I guess this was just mre of what was knocked loose when we cycled the kerosene. My son then successfully then drove it 4000 miles across the country, with no oil messages or lights coming on. So again, I am not telling anyone to use this method, just telling you what worked for us. If you read about using kerosene in an engine you will find all kinds of horror stories, but those are usually from people who have never actually done it, just regurgitating what they read from others telling stories. I read that after using kerosene every seal would fail and oil would pour out from every gasket. This did not happen. I of course didnt crank it all the way through the engine, and I didnt leave the kerosene in the engine longer than necessary.

You could have used ATF instead of kerosene. Then you could have idled the engine for a couple of minutes at a time, with a filter in place! Or gone the route of an expensive engine cleaner. But not having a filter in place did not buy you anything, it just made the kerosene dirtier. You might even have caused a bit of rubber to get stuck in a block passageway.

Don't blame the PO for the sludge, most shops in 2006 used conventional oil, and if he drove it to work he probably never got the engine oil up to operating temperature. I think our local Volvo dealer didn't start to use synthetic oil until 2015, cheap bastard...

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Keeping it running is better than buying new

I was not trying to clean the engine. Yes, if that was the intent I would have had a filter in there, and I would certainly NOT have run the engine with kerosene in it. I even removed the fuel pump fuses to be sure it did NOT run. I am aware there are all kinds of engine cleaner snake oils out there, but again, I wasn't looking for that. I was trying to resolve a low oil pressure issue/light. The process I outlined successfully did that. It dislodged the particles/debris that had clogged the pickup screen. There was NO kerosene in the oil filter housing after the process I did, so a filter did not matter. This tells me the kerosene did Not circulate though the oil system, which was the intent. Just wanted to clarify that as I am trying to help others who might have a similar situation. The car has been happily motoring for a cross country trip/5000+ miles since with no oil warning message.