For what it's worth, I just switched to Pennzoil Ultra 5W-30 in my 1997 Jaguar XK8 Convertible. I find the engine runs smoother and acceleration is also smoother and better than before. My car has 86,000 miles and I have used Mobil 1 5W-30 for about two years and Valvoline Synpower 5W-30 prior to that. I don't know what is different about this new oil, but there is no question I feel a difference.
Scott W. Moseley
1997 Jaguar XK8 Convertible
2001 Jaguar S Type 4.0
1990 Jaguar XJS Convertible (for sale)