Well I'm stumped. I got my S70 out of winter storage and took it for a ride to check things out and as soon as it warmed up the oil pressure light came on. I checked the oil and it was full. Did a search the Brickboard and learned all about the "O" ring problem causing low oil pressure with S70's, so I planned to put it away and planned to replace them along with the timing belt. (109,000 miles on the clock) I completed the job and took it for a test drive and and as soon as it warmed up the light came on again. "O Crap"! I can say that I really didn't find any problem with the "O" rings between the pan and block and didn't think I was fixing anything by replacing them and the other seals in the oil pick up and cooler on the rear of the pan. One note I would like to mention about the Oil Sump "O" ring kit. Out of 9 items in the kit only three were useful on my S70. I called FCP and was told that it is the same kit put together by Volvo for all 850 - S/V-70 cars. It needs to be updated in my opinion. There is a pipe in the oil pan between the filter outlet and oil cooler that has a seal on both ends. Both were cracked and hard and I had to replace both with ones from my "O" ring kit. Two large "O" rings on the oil cooler were not also included. There is no place to use the flat gaskets. The whole kit is a rip-off.
Back to my problem. I removed the oil pressure switch and tested it. It is a normally closed switch contact that opens at >10 psi. Conclusion, sensor OK. I replaced it with a pressure gage and was surprised to see the oil pressure at 65 psi with cold engine but as it warmed up it gradually dropped to <10 psi. Increasing RPM did not raise the pressure.
So that is where I am at today. I did some searching to try to find the oil pressure relief but only found it mentioned in a list of things to replace if the oil pressure is too high. It cannot locate it though. Does anyone know where it is?
Someone on the list was recently having the same problem. Was there any resolation?
Of all the low oil pressure problems I found in the search I did not find ONE that indicated that replacing the "O" rings fixed the problem.
A little history on the car.
I got it on Ebay from New Orleans,wanted a southern car. It had 65K miles on it and was not treated very well by the PO. The engine was quite sludgy, indicating few and far between oil changes. I found a lot of sludge in the pan when I removed it. It runs and sounds good except for the noisy valve train which I thought was a result of the low pressure.
Sorry for the long rant, but I'm lost. Going to pull the pan again today to see if I messed up the "O" ring installation.

--
Tom 69-1800S, 72-1800ES, 96-850T,2000-S70 GLT-SE