Funny thing about this irregular dipstick reading - many XJ40 owners mention it, yet I've never heard what Jaguar themselves have to say about it...

 

I have found my car's oil reading to be surprisingly affected by any angle or slope in the road where the car is parked, versus a correct reading on a completely flat road. If the road slopes even slightly I find the dipstick reading will under or over read, depending which way the car is pointing.

 

I have the same manual for my XJ. Love the part where it says 'M' line. Good luck finding it on the dipstick.
I find the best way to compare oil levels from week to week is to check it with a warmed up engine about a minute after shut down.
There has been some comments on the oil filter non-return valve. I believe the OEM oil filter HAS a non-return valve but do all aftermarket filters have them?? This can change the oil level albeit only slightly.

 

Several aftermarket filters have been reported as not having a non-return valve. They are 'compatible' which means little more than they fit!

Graham