Hi Ben
Your normally aspirated XJ8 would take 6.9 quarts without an oil cooler, or 7.9 quarts with an oil cooler. For your part of the country ( temperature range ) Jaguar recommends a viscosity of 5W-40.
I have used a full synthetic either Mobil 1 or Castrol Edge OF THE SAME VISCOSITY in my 2002 XJR with great success. I tend to stay with Mobile 1, it is a good all year round weight oil in all temperatures up to 120 F. NOTE- always check the dipstick after replacing your oil and filter and at least once a week
Happy Jaggin
