There might be a lot of reasons that the oil-pressure is low, but because it happened after an oil-change it’s likely to believe it is related to this.
First of all, the white o-ring inside the cover serves to purposes:
1) It seals so the oil would not leak out of the cover
2) It seals so the oil would not leak from the canister (the pipe you put the oil-filer inside) to the sump.
#1 is pretty easy to spot because there will leak oil around the cover, #2 is impossible to see and it will reduce the oil-pressure.
The O-ring has to be compressed to work; it is 4mm thick and should be compressed to3.0 – 3.3 mm. The way to measure this is:
If you measure more then 3.3 mm you have to use shim(s) to increase the compression on the o-ring (shim=0.3mm). If you use the papergasket you reduce the compression on the o-ring and you might get a bigger internal leak (#2) but you want know it!
If you measure less then 3.0 mm (not likely) then it’s okay to use the papergasket (or you might crush the o-ring) but otherwise don’t use it because it camouflages internal leaks.
It’s smart to change the o-ring once in while.
So why bother about this when your engine has been working for 95000 miles?
Because the canister tends to move over time, mine has moved twice and the last time the distance was 4.6mm!!
If you have this problem (or want peace in mind) you can get a new canister made in the correct length so it bottoms in the engine (no place for movement).
Of course before you do this you check that you have filled the right amount of oil with the correct viscosity and of course that it is the oil-pressure warning light that lights up.
Other possible solutions could be:
Worn oil pump
Worn housing around oil pump
Front main bearing out of position
Worn bearings (main and conrod)
Faulty cam-chain tensioner
Most of these errors develop over time, so I would check the canister and o-rings first.
I like to fit an oil-pressure gauge on all my vehicles and see the variations over time. Then I know what’s normal and can start to worry if the pressure drops. It’s hard to know afterwards, you might measure 3 bar at a specific rpm and temperature but how do you know what’s right?