Done a quick search, not found much really. (sorry for the essay btw)

Have a Oil pressure gauge fitted to the car and after initially starting up the car after an engine conversion, oil pressure registered at about 60-70 psi dropping to about 35-40 psi (on the gauge) when warm (I think, memories a bit dodgy)

That was a few months ago and not long after that had a massive oil leak where the oil filter was fouling and wore a hole in the filter meaning oil was being sprayed everywhere and losing oil pressure (gauge down to 10psi and lower I think, whatever it was, the oil pressure light on the dash lit up telling me there was a big mess in my engine bay and the following cars windscreen as I got told)

Replaced with smaller filter; not sure if its since then or some time after but the oil pressure has been registering upwards of 100psi, only dropping to 75psi when idling and engine is roasting comfortably.

There's also been an oil leak coming from the oil separator (?) underneath the exhaust manifold. Thinking it was the gasket, I finally got round to changing it a few days ago and found that the positive crankcase value breather pipe had perished badly.

So new gasket, new breather pipe but yet again started leaking.

Someone mentioned it could be caused by High crank case pressure possibly forcing oil out the gasket, but running the engine with the positive crankcase ventilation valve off doesn't seem to lower pressure at all, though it does seem to idle a bit better.

Been told to try the pressure relief valve, will be doing that tomorrow (if I can find it), but is high oil pressure bad and is there anywhere else I should be checking?

What was the larger oil filter fowling on? Personally i would try a decent engine flush (i'd recommend Forte or wurth), run it round with that in the oil for a day or so, drain the sump over night fill with fresh quality oil, i would also clean out the sandwich plate (thats how im assuming you have an oil pressure gauge fitted) incase it was a blockage causing a faulty reading before fitting a new filter.
If fhat problem still persists i would cure any oil leaks in the bottom end (although a leak would lower the crank case pressure) and clear all the engine breather pipes or replace them

The back of the engine bay, theres some kinda of bar or support or something; everytime I put my foot down, engine would swing back a bit and hit the filter.

The gauge is fitted with T-piece majig where the oil pressure switch is mounted (back of the block)

Had a fiddle with the pcv, took it out and gave it a clean with petrol, not improved at all. Normally starting the engine with it attached, I'd have to press lightly on the throttle because the car won't idle on its own until its warm; but with the pcv off the car will start up perfectly :/ So think I may have found problem causing the difficult idling.

Bought some flushing oil this morning and a new filter so will get on with that in a bit and see how it goes

*sigh* So drained oil, stuck some flushing oil in; reved the engine a tad above idle ( with the pcv out) and after a min the car starts stuttering and struggling. Kept reving it slightly above idle and suddenly comes some loud pop or bang noise from beneath me. Scared the s**t out of me whatever it was.

So car stalled, left it a min and started it up and it idled fine. reved again and same problem, bit of stuttering and what not. What a pain in the a*se

*Edit*
Okay drained flushing oil and stuck in some Fully Syn 5/30 oil and a new filter (which was even smaller again then previous). Unhooked battery to reset ecu. Ran engine with everything where it should be and at first, car idled perfectly.

Once the engine had warmed up problems started, car started stuttering again, found it difficult to idle and oil started leaking from the oil separator again despite the new gasket and care given when assembling it all.

Couldn't find the pressure relief valve where the haynes says it should be, so guessing its in a different place on a 2 litre zetec engine then the 1.6. Did notice that the filter housing was different on the 2.0 lump then the 1.6 when converting but thought nothing of it really.

Does anyone know where the oil pressure relief valve should be on a 2l engine? Hoping it doesn't involve dropping the engine down to access the oil pump because I'm starting to wonder if this car is worth it with the amount of problems its had in the past