Hi, i just bought a 1997 Jaguar XJ6 w/ 93,000 miles on it. It has a 4.0L Straight 6 engine with aluminum block & heads. I do not know what the previous owner used in this car, but it had a Jaguar oil filter on there. Currently I put in 10W-40 Valvoline MaxLife. I will be taking it down to florida in about 3 weeks. Once the car gets to florida, where the temps will never be below 40*F, which would be the best mineral oil to run in here (HDD 15W-40 oils have had good reviews). This car has an 8.5 Qt. sump capacity and I plan on putting on a Motor Guard too. So i don't think synthetics will help that much especially with the motor guard on there keepin the oil clean.