Hello:
I drive a XJ6 1997 model with 81,000
I have recently noticing that If I'm driving in a straight line and turn the wheel ever so slightly to the right, I feel something in the wheel, not sure if it's something rubbing, or a vibration, (not sure really how to explain it) the steering wheel is not shaking, the car is not doing anything weird and I do not hear a noise, but I'm feeling something in the steering wheel. My steering fluid reservoir is full. Any idea what might be going on?

Regards.

 

Not all but some of the X300s have a variable steering triggered by the speed of the car

To know since yours is a left hand steering there is a variable steering amplifier behind a side panel of the drivers left foot

THis can be seen on page 19 as it is unmarked in the picture in front of the 2 front windshield electric heat relays which you probably don't have ( expensive option )

X300 1996 LWB.pdf (jagrepair.com)

 

I would give your front wheel bearings a quick check. Jack the car up, and check the road wheels for play across the 12 oclock / 6 oclock axis . There should be a very very small amount of play.
Counterintuitively, zero play is not a good thing.