The low oil pressure warning light is staying on when the engine is started it glows bright then goes dull but doesn't extinguish completely, I'm hoping this may just be the sender unit not sure.

obvious question... (hope u already checked)

is theyre enuf oil in your sump

when motor is off. pull out.. wipe it with a rag.. drop it back in and then pull out agen and see where it is according to your guage on the stick.

if its below Empty mark.. thats the simple solution.

otherwise your oil pump could be blocked and not enuf oil going to the motor. thus causing the sender to say its low.. or the sender could be broken.

How long since the oil filter was replaced?

I did an oil change this morning using a new filter and the warning light is stilll on, are the senders in the same location on XR6 and 3.9 engines the original sender unit from the 3.9 has a small type body where the XR6 has a longer body?

Mate that could be your problem. Sounds like you have the gauge type fitted to the engine when you only have a light on the dash. Yes the senders are different usually the light ones are smaller than the gauge type and I am pretty sure you can swap them over just put some thread saeler on the thread before installing.