WARNING MESSAGE sTOP: OIL PRESSURE ...wHAT IS

Not good. It is low oil PRESSURE, not low oil LEVEL, although low oil can cause low pressure if the pump is scavenging. When was the last time the oil filter was changed?

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The low oil pressure warning lamp will illuminate anytime the ignition switch is turned to the ON position without the engine running. When the engine is running, the light also illuminates if the engine oil pressure drops below a safe oil pressure level. To test the system, perform the following:
If the oil pressure light does not work, perform the following:

1. Turn the ignition switch to the ON position.
2. If the lamp does not light, check for a broken or disconnected wire around the engine and oil pressure sending unit switch.
3. If the wire at the connector checks out OK, pull the connector loose from the switch and, with a jumper wire, ground the connector to the engine.
4. With the ignition switch turned to the ON position, check the warning lamp. If the lamp lights, make sure the oil pressure switch is properly grounded in the cylinder block, and if so replace the oil pressure switch. If the lamp still fails to light, check for a burned out lamp or disconnected socket in the instrument cluster.

To diagnose an oil pressure light that stays on, perform the following:

1. Check the engine oil level and top off as necessary. If the engine was 2 or more quarts low on oil, test start the engine after topping off the oil level and recheck the oil pressure light operation. If the light stays on, stop the engine immediately and go to the next step.
2. Disconnect the wire at the sending unit, then turn the ignition switch to theON position. If the light remains on, check the wire from the sending unit to the gauge for being shorted to ground and repair as necessary. If the light goes out, turn the ignition switch OFF and go to the next step.
3. Replace the oil pressure sending switch with a known good switch.
4. Start the engine and check the oil pressure light operation. If the light stays on with the engine running, stop the engine immediately to avoid severe engine damage until the cause of the problem can be determined.

WARNING
If the oil pressure light remains on while the engine is started, stop the engine immediately. Do not operate the engine until the cause of the problem can be determined. Operating an engine with low oil pressure will cause severe internal damage.

Possible causes for an oil pressure light to stay on are:


Pinched or grounded sensor wireDefective oil pressure sending unitRestricted or severely leaking oil filterExcessive internal engine bearing clearance.Insufficient or severely contaminated engine oilBlocked oil pump pickup screen or severely worn oil pump

First, take it to another mechanic....

If you get an oil pressure warning, then one of two things is wrong:

1. You don't have oil pressure. Could be oil pump, or other internal, expensive problem.

2. Oil pressure sensor bad. Easy to replace.

That depends on the type of oil warning light being displayed. There are 2 types of warning lights

1. Low oil pressure light which all cars have
2. low oil level light which some cars have.

Low OIL PRESSURE warning means the oil pressure is below minimum and when this happens there will be insufficient lubrication of many engine parts because sufficient oil cannot get to all parts of the engine it is required to lubricate. Until the cause is identified and rectified the warning light will remain. If you run your engine with low oil pressure you will cause expensive damage to internal engine components.

Low OIL LEVEL warning means the oil in the sump has dropped to a dangerously low level (which may be due to an oil leak or abnormal oil consumption by the engine.) This warning means the oil level needs to be topped up and the warning will stay displayed until you top up the oil. Running the engine with low oil will cause expensive engine damage.
Since it is a 2 minute job, Oil levels should be checked fortnightly as a minimum, together with radiator coolant and power steering fluid levels, if you run the car most days each week as most people do.

It is possible for a warning light to remain displayed when the cause has been dealt with because there is a problem with the sender unit. However it needs to be established with alternative checks that the problem has been fixed before investigating if there is a problem with the warning light itself.

The engine bearing clearances expand with heat which, if excessive, will lower the oil pressure. The spring in the pressure regulator in the oil pump will cause the pressure to drop for the same reason. A partially plugged oil intake screen will work in conjunction with the oil pump to cause it too.

Without any more information I'm going to simply "spitball" an answer.

It COULD be oil starvation caused by high mile/kilometer sludge in the engine which slows the return of the oil to the pick up screen of the oil pump. Starving the oiling system.

It COULD be low oil level which would cause the oil pump to starve.

It COULD be a faulty sensor/sender causing the warning message.

Being you said "on the motorway" I'm simply guessing this is the only time the happens and NOT when driving at slower speeds/RPMs.

Hope this helps.

The pressure regulator in the oil pump may not be working, the idle speed may be too low and there may be a lot of wear on internal engine parts lowering oil pressure

It's a warning light system that no oil pressure exists. If it doesn't work, it won't cause damage, you just won't know if and when your oil pressure is lost. The sensor or sender unit is very inexpensive on most vehicles, like under $10.
All the sender does is cause the oil light to go off when it detects minimal pressure. A valve inside the sender moves and causes the sender to stop grounding the warning light, and so the light goes off.

Get it in for diagnostics, the possibilities are endless, but do not drive the car!
A) Check the oil levels, and if there are any leaks
B) Check oil pressure sensor

When last was an oil change service done?

If you have low oil pressure you should have a waring light or message, the engine will become noisy and if it is not repaired very soon you will here first a loud tapping noise then a loud knocking deep in the engine, and finally the engine will worst case blow up and parts may come out the bottom through the oil pan. Never drive any car that shows a low oil pressure warning light or message, stop the engine right away and have it checked by a shop.