What engine does the car have?
I hate to say it, but it sounds like you bought somebody else'* problems. I bet it was making noise before, you throw 20w-50 in there, with lucas oil stabilizer in there, and the noise goes away, at least for a while. That jacks the oil pressure sky high (leaking oil pressure sensor is not surprising at this point) and shuts the engine up. If it is a v6, I suspect the extended life coolant was never serviced, and ate away gaskets, and dumped coolant in the oil. If it is a 4 cylinder, it could be cam noise also.
Also, a quick search tells me that you also had a plugged A/C drain, as well as an airbag deployment on the last owner due to a shorted airbag module. Based on how often inspections are in your area, I wonder if those are related as well.