Another Oil Pressure Light Question

[Gnarly Old Dog]

Hi Guys, Sorry but I need some help please. I've searched the Wiki, read other articles and tried to refrain from posting yet another oil pressure light related question but to no avail - I need some help please.
Long and the short - I'm prepping my bus for it's MOT after it's habitual hibernation over the winter where it's SORNed and off the road. Started it for the first time in nearly 5 months at the weekend with no dramas - in fact, it was better than expected. Took the bus for a drive tonight and all went well - I even started to smile as I circled around the town for a second spin - and then it happened. On down-shift approaching a roundabout the oil light flashed as the revs dropped to idle. It's done it before on really long journeys when everything is warm but not just around town on such a short run. When I got home, again, at idle, the light comes on - no buzzer - just the light which goes out once you accelerate to around 1500 rpm (below the 2000rpm Lo/Hi switch point)
It's a late 1990 1.9DG so it has the BOD twin sensors and I know that the BOD works as a faulty earth nearly had me soil the seat a couple of years back on the A303.
So - self diagnosis - I reckon it's either:
a) faulty 0.25 bar low pressure switch due to old age
b) faulty Oil Pressure Relief Valve - possibly gummed up from being sat around all winter and me not quite getting around to the oil change before the lay up (oil is approx 18months old / 2000 miles)
c) partially clogged oil filter due to above mentioned non-service before lay up
Could be a poor earth but the intermittency (it didn't do it until it was warmed through tonight and didn't do it at all at the weekend) seems to lead me to think it's possibly a combination of b+c so I've ordered all the parts to do the above plus the overdue oil change (already had a Mann filter - just need to get the oil)

Basic question really - Am I likely to cover myself in oil if I remove the OPS before draining the oil or is the OPS above the oil level when the engine is non running?
Same goes for the Pressure Relief Valve - I'm guessing though that as soon as I undo that big threaded screw that the oil will drain through that as readily as if it's the sump plug?

I know they're really basic questions but I'm panicking a bit with the MOT already booked in and I don't want to chance the fact that it's just a dodgy warning system in case it is a proper low pressure issue.
Oh BTW - it's done about 225K Kms (140K miles) and is, AFAIK, the original engine. I know it's old but it's been pretty reliable and no signs of smoke or dropping oil other than the normal territory marking that they all seem to do.
Thanks for any help and pointers...
Andy.

[what2do]

Others more informed than I will comment soon. My input is as follows, why concern yourself about getting covered in oil? Drain it, replace the sensors that you've purchased and refill with fresh oil. It certainly won't do any harm by changing your oil.

[dsmith1974]

Agreed. I had low op indicator at idle (when hot). Drained the oil, replaced the switch and put in a new filter and 20-50. Pleased to say it's been fine since so I'd probably try that first.

[chriscburgess]

Did my oil pressure switch yesterday without draining the oil. Only a minimum amount of oil came out. just put down some newspaper and a tin to collect the oil in. have a rag to stuff in the hole in case of any problems.

However, I too have a question - is it possible to change the filter without draining the oil?

[what2do]

Did my oil pressure switch yesterday without draining the oil. Only a minimum amount of oil came out. just put down some newspaper and a tin to collect the oil in. have a rag to stuff in the hole in case of any problems.

However, I too have a question - is it possible to change the filter without draining the oil?

Are you just curious or genuinely need to know? Why would you be needing to change the filter and not the oil?

[chriscburgess]

Did my oil pressure switch yesterday without draining the oil. Only a minimum amount of oil came out. just put down some newspaper and a tin to collect the oil in. have a rag to stuff in the hole in case of any problems.

However, I too have a question - is it possible to change the filter without draining the oil?

Are you just curious or genuinely need to know? Why would you be needing to change the filter and not the oil?

Did an intermediate (6 monthly) oil change without changing filter but now wish i had changed filter so wondered if it was possible without losing too much of my new oil?