I have low oil pressure on my series 1.5..(10 psi warm at idle, 25 when driving)....Im going to change the oil soon and ive seen in another post that if i change the pressure relief spring this might increase the oil pressure a bit..so my question is how difficult is it to change it out? In the exploded diagram it looks like i remove the 4 bolts that hold the oil filter housing to the block, and then i disassemble it? Is anything pressed in? Should sealant be used on the gasket that goes between the housing and block when i put it back together? I cant find much info on any forums or you tube on this...

It’s an easy job, but I am really not convinced this will solve your issue, unless the valve is stuck open... plenty of other reasons for low pressure. Get a proper mechanical pressure gauge with suitable range and plug it in the current pressure point. Do not rely on the dashboard gauge.

For the needle valve; basically take off oil filter, then the whole assembly on which the oil filter is mounted (4 bolts from memory). The rest is self explanatory when you see the parts.

Good luck

Apart from the electric gauge not working properly, you could just have a worn engine ! If you don't want to do a full strip-down and rebuild, a new oil pump might do the trick. Also use the correct viscosity oil as orginally recommended. Jaguar engines never had very tight tolerances, the machinery they were made on was secondhand when installed in 1948 !!