I would like to know where can I get the WSS-M4G320-A3 sealant or what sealant is compatible. I can't seem to locate it. This is for the 2002 XJ8 valve covers.

 

A little dab of RTV is all you need at the cover/block seams.
I use Hylosil or VW oil pan white RTV sealant. It's kinda pricey but made for oil sealant.

You could use any RTV that you have around and it should work OK.

bob gauff

 

Jaguar recommend 'Loctite Ultra Grey' if you can get it locally, although Bob's suggestions are probably as good. Just ensure all faces are minty clean on assembly.