It is useful to know that there are aftermarket solutions to a failed sensor out there - it is a very common XJ40 problem.
My own car suffered an intermittent sensor problem where it would flicker from 1 to 2 on the gauge at hot idle. Surprisingly, I solved the problem by cleaning off minor corrosion from the connector with very fine sand paper. This was several years ago - and it has worked perfectly since. This probably won’t work for everyone - but it’s worth a try!
In the dim recesses of my memory I seem to recall there was someone who found a GM sending unit that worked with the Jaguar gauge with no need for a resistor, at least no mention of a resistor was provided in the post from at least 5 or so years ago. The sending unit was sourced from a local auto parts house.
John Quilter
Eugene, Oregon USA
Hi John, I found the posts you referred to in the XJ40 archive, one of which was a post by yourself back in 2011 quoting the original post from 2005 by bsparks72, which referred to the AC Delco part E1800A.
This unit, if available, will run you in the range of $150+ US …however cross-referencing this part number you get the equivalent Alternate/OEM Part Number(s): 88924425, C46272, DAC11141, DAC11141K, DAC7879, DAC8817, DBC4418, DBC5513, JLM20791, LMD5640AB etc.
The AIRTEX/WELLS 1S6678 or BECK/ARNLEY 2011505 units also cross-reference to a similar set of alternates, C46272, DAC7879, DAC8817, DBC4418, DBC5513, DBC8817, LMD5640AB etc. These units seem to be physically identical to the Delco part and cost about $38 on average, although more gullible folks can pay $150 for these too on eBay.
I should mention that fitting the Airtex or Beck/Arnely units without a resistor does result in a somewhat “satisfactory” reading, if a little “too satisfactory” 
- they read a mite too high, almost, but not quite, pinning the gauge in the cabin on cold startup and after warm up giving a reading of somewhere between 1/2 and 3/4 up the gauge.
One can run like this, it just registers a “bit” higher than actual …and that is no doubt preferable to having a zero reading or the low oil pressure light flickering at a traffic light.
I venture to guess that this is possibly how the OP, bsparks72’s Delco unit worked?
Anyway, IMNSHO, fitting the 75 ohm resistor to the Rockauto (no affil) equivalent would be the best solution for today’s XJ40 owner - it would be a pity to fit the pricey AC Delco unit and find it reading high and needing a resistor when for about a quarter of the price you could get the same thing.
BTW, if you fit the idiot light/switch as fitted to the X300’s. you need a 150 Ohm resistor to centre the cabin gauge.