Hi Nicrane, Derek and Larry,
You are spot on about the 'sixes' being bullet-proof as regards oil pressure - not only do they tend to run really good oil pressure, the bottom end in general seems to be very long-lived if looked after with regular oil and filter changes. I know of '40s with over 400,000ks with a couple of reconditioned heads during that distance that still use no oil!
The V12 is a very different beast though and is notorious for running very low oil pressure (particularly at hot idle) so I would never recommend running a 12 without a working oil pressure gauge - particularly on the 40/81 where there is no separate circuit and switch for the 'idiot light'. A 6.0 is streets ahead of a 5.3 when new, with around double the hot oil pressure at idle due to running tighter bearing clearances, but neither are exactly world-beaters. In a road car the V12 seems to survive ok despite its low oil pressure at idle, but it is important to make sure the needle heads smartly upwards as soon as the engine revs rise above 1000 rpm and it is for that reason I would always recommend running a working gauge in a 12.
I'm not sure if Darren's car is a six or a 12, but hopefully there is enough info in all of our responses to cover his needs either way.
'94 XJ12 (and other toys)