Ok, I have the 1999 XJ8L
4 litre..
Alldata shows I should have the motor with the firing order of Drivers side of engine being bank 2 and 1,2,3,4 and passenger side is bank 1, 1,2,3,4
Like...
drivers side ------ passenger side
1 (really 5) 1
2 (really 6) 2
3 (really 7) 3
4 (really 8) 4

But if the motor is 2002 or newer its...
drivers side passenger side
2 1
4 3
6 5
8 7

-------------
Question is if I have code 306 is that truly the 3rd cylinder on the drivers side, or the 2nd cylinder on the drivers side?

 

Here is the cylinder numbering diagram.

For your car 6 should be second from the front on the (US drivers) left side of the car.

 

It is important to remember that the firing order did not change, only the numbering scheme.