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.