1995 Other Jaguar Models temperature gauge reading error?

Hi, i bought a 95 Jaguar XJ12, 83333 mileage, engine runs strong, transmission shifts smoothly, temperature gauge always at N (reads Normal) if not just slightly above it. Recently, i had an oil change and soon i drove off the auto service lot, the temp gauge went all the way up! Almost hit the red line (H). I don't see smoke coming out of the hood, and i made it home without the car breaking down, i opened the hood and i didn't smell anything. What causes this? Fyi, when i'm on the highway, the gauge fluctuates somewhere between N and one bar above it but never hits the H. Ive been using 10w 30 oil but that day i switched to 5w 40 synthetic. Could that be the problem? Is my engine really overheating or is it just the gauge problem? Any info would be appreciated. Thank you so much!

cheers,

proud jag owner