Oil pressure light comes on when i've been driving for a while, have changed oil pressure switch and the wiring to it and still the light is coming on! am wondering if it is the oil pressure relief valve? could someone give me advice please?

Try shorting the oil pressure switch to earth (i.e. to battery -ve).
I think these motors shut off if you switch the oil pressure sense wire to the chassis, which simulates a loss of oil pressure (oil pressure warning light comes on).
I don't think it prevents it starting, but it shouldn't run when you release the starter. Cheers.

Are you calling the oil pressure switch the oil solenoid ?
The oil pressure switch could have failed which would push oil out the terminal where the wire is connected.

I can't imagine anything being affected by changing the oil other than the valves in the heads. I assume you have good oil pressure hot or cold. I suppose you could be moving a wiring harness around when you take the oil filter off.
It would depend on if the rough running is mechanical or fuel or ignition related to say what to check first. I would have to figure out which area is affected first.
Something like a sensor or a dead miss on one cylinder could turn the check engine light on, but most fuel and mechanical issues will not do it on a car of that year.
If one of the coils was failing when the engine is hot, changing the oil should have no effect.

Could be your oil pressure switch,here's a link.http://www.autozone.com/autozone/parts/Duralast-Oil-Pressure-Switch/1985-Toyota-MR2/_/N-ijxzlZ8znc6?itemIdentifier=261416_0_8450_ Here;s the location.http://www.autozone.com/autozone/repairinfo/componentlocations/componentLocationMain.jsp?fromSearchPage=true&categoryNValue=4294963411&categoryName=Oil+Pressure+Switch

I'm guessing your battery charge lamp and oil pressure lamp are coming on when you shut down the engine. This is perfectly normal because your battery is no longer charging and your oil pressure is down because your engine isn't running. If however you've switched the ignition off and these lights appear briefly while the engine is stalling, the fault is either a diode in the alternator, or a wiring fault within the dash display. Hope that helps.

The first order of business is to check the oil pressure while the gauge is not working. If the oil pressure is fine and the system reading zero, there is a fault in the oil pressure switch circuit. Often, the problem is related to the oil pressure switch itself. I have seen cases, too, where the oil pressure switch fails internally, causing oil to push past the seal and into the wiring connection causing a short. The wiring harness has a single pin.

Another area, though it fails much less often, is a break in the wiring between the oil pressure sender/switch and the ECM module (located in the right passenger footwell) and between the ECM and the instrument cluster. The instrument cluster may also have a short, though the sender is the most likely issue here.

-Ed
ASE Certified Master Technician since 1984 - Everything from toy cars to the largest of trucks
Consultant for the North Carolina Consumers Council

This could be caused by a number of things. I've seen everything from low oil level (which is what you should check first), to a faulty sensor or loose wire cause this. If when you check the oil the level is fine, get a buddy to sit in the car and watch the light while you wiggle the wires that attached to the oil pressure sending unit. If the light flickers, you know you've got a loose wire in that connection and it will need to be looked at. Definitely check the oil level first...this is the most common cause. When you come to stop or turn a corner, the oil in your oil pan sloshes around, back and forward, side to side. Depending on where your oil pikeup tube is located in the pan, this can cause a small air gap which will utterly demolish your oil pressure causing the light to come on. Since the oil is sloshing back and forward this would cause a break in pressure, followed by a return of pressure, followed by another break respectively depending upon how low your oil level is and how often the tube sucks in air. It seems really simple but definitely check that first. Let me know if this doesn't work out and I'll see what else we can come up with.

Oil pressure is always lower at idle speed.I'm guessing you have high mileage.If your oil light is not coming on in drive at idle with the engine cold,but it does when hot,your bearing clearances are probably getting a little loose.Try using one quart of Lucas oil additive in place of a quart of oil at your next change(or just change it now).This stuff is thick and stringy,it helps the oil cling to surfaces.(if there is no change after this,you could try a new oil pressure switch-it's cheap enough.I actually had the switch on my 95 dakota leaking a LOT of oil at high pressure/RPM,which affected the reading).

check your distributor pick up wires, make sure they are not crossing.