Jaguar Repair: 1986 Jag XJ6 - oil pressure, hydraulic gauge, oil shop

Question
Me again..I brought my stored jag of 7 years to get an oil change.  I had been driving it for a few days and everything was running smooth.  After a day or so following the oil change the oil warning light started to come on in an idle state and would disappear while drving, now a few weeks later the light is on constantly, idle or driving.  Question is can the grade of oil effect the pressure.  I had always used 20w-50, then the oil shop suggested 5w-30, synthetic for the older car (88,000 miles).  Wondering if the grade is causing the pressure problem? or if there is other causes to look for at this point?  Thanks Much

Answer
Yes, the thin oil can make the oil pressure low, however the idiot light and the gauge are electric and should be suspect. You need to get a hydraulic gauge to test with and put it in place of the oil pressure light switch (the small switch) and read the actual pressure with warmed oil. 10w-40 or 20w-50 should be used depending on temperature and mileage on the engine. You should see from 25 to 35 PSI at idle with warm oil and 40 to 65 PSI at cruise speed.

If you have less than that with the proper oil you may have a problem.

Howard