The oil pressure gauge shows even lower pressure than last year...
Yesterday I was not able to pass 30 at 2500rpm/ 80km/h. At stillstand/idle the pressure sinks to just above zero. I know the senders can be unreliable, so is there an easy way to check the real/physical pressure? What is "normal" pressures at different speeds/rpm and engine temperatures?

 

I read somewhere that 20lbs at idle was Jaguar suggested pressure and 40lbs at 2000 rpm. Have you changed oil lately? You can check pressure by buying another gauge and trying the different oil galley ports along block or just to check dash gauge. Could check oil filter base and relief valve for sludge.

 

20 and 40 appears more reasonable. Last summer there was done flushing/cleaning + all new oil, new filter/gasket/return hose. So it should not be gunked up there after 2500 km. Of course the engine is leaking, I usually have to refill +/-5dl (pint) about every 1000 km. There is no blue exhaust smoke, and no excessive dripping om my garage floor, so I really don`t know where the oil is going, Maybe it gets gunked up in the oil pump? I should have someone do the physical pressure check directly on the engine block as it is the sender, not the gauge itself that usually get erratic over time.

 

Might not be sludge, just the idea, even with flushes things stay in sump. You might check the relief valve in the oil filter block for debris. Things like deteriorating oil filter, seals , etc. These can get in the relief valve and open enough to cause pressure loss. When engine is flushed, not all the ancillary lines get cleaned. Always do a 2nd oil change after a flush and fill. The older Jags have round panel in the bottom of block, this is an access to clean oil pump intake screen.

 

It is normally the oil pressure transmitter mounted atop the oil filter housing that gives trouble. A pressure gauge can be mounted on this to give a "real" reading. Other thing to be aware of is that as the miles mount up these engines can start burning oil but without obvious smoking, but get a friend to follow you for a bit when the engine is warm to see what comes out.

There are seals on the inlet valve guides that eventually get hard and fail to seal. Modern "thin" oils can find all the escape routes too.