I use Castrol full synthetic 5/30 in both of my Kitties, the S-Type V8 and the XJ8 every 5k. My 02 XJ8 now has 80k trouble free miles and the S-Type V8 50K. The 2003 XJ8 is by far a more superior car than the S-Type, it is simply built much better

 

last oil i used was a castro synthetic and i only run on premium, my car shakes at idle or after start up in the morning when i use mid range.

 

Doesn't sound to be due to oil, but does sound like a fault. OBD readings (not just codes) should help.

 

I believe the cars have similar engines, no?

 

Very. The XJ8 may still be the 4.0 but the engines are very alike (with more differences than may be initially thought).

I think he means the body stuff etc.

If he has the original V8 S-Type then he's missing the many improvements when the car was redesigned (and engine went from 4.0 to 4.2).