When i am driving on the free at speed of 70mph a message come on the screen to check the engin oil next refuling yet the oil level is fine

Replace the instrument cluster . GM had stepper motor issues 2003 to 2006 . Pin out won't help you , The info for the instrument cluster gauges come's over class 2 serial data buss (network) from the PCM - engine computer . Do you know what GM class 2 data network is ? All gauge info comes over one wire from the PCM .
How to repair GM Instrument Cluster Speedometer Gauge Stepper
Buick Class 2 Operation Diagnosis 1998

Hooking up a Tech 2 scan tool an checking for code's an data to the instrument cluster would be the correct diagnostic procedure . But you wouldn't know that , not being a Chevy technician . Not having factory service repair info. manual .

Speedometer
The IPC displays the vehicle speed based on the information from the PCM. The PCM calculates the vehicle speed from the vehicle speed sensor (VSS) circuit. The IPC receives a class 2 message from the PCM indicating the vehicle speed. The PCM also calculates the vehicle speed based on the wheel reference vehicle speed class 2 message from the EBCM. The PCM uses the vehicle speed information from the EBCM as a backup. The IPC displays the vehicle speed information from the EBCM when the PCM detects a malfunction in the vehicle speed sensor circuit.
The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of class 2 communications with both the PCM and the EBCM.

Engine Coolant Temperature (ECT) Gage
The IPC displays the engine coolant temperature on the analog ECT gage as determined by the PCM. The PCM monitors the signal circuit of the engine coolant temperature sensor. The IPC receives a class 2 message from the PCM indicating the engine coolant temperature. The IPC illuminates the check gages indicator when a high engine coolant temperature condition exists. The engine coolant temperature gage defaults to 75°C (160°F) or below when one of the following conditions occur:
• The PCM detects a malfunction in the engine coolant temperature sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
For additional informatin on the engine cooling system, refer to Cooling System Description and Operation in Engine Cooling.
Engine Oil Pressure (EOP) Gage
The IPC displays the engine oil pressure on the analog EOP gage as determined by the PCM. The PCM monitors the signal circuit of the engine oil pressure sensor. When the engine oil pressure sensor resistance is high, the engine oil pressure is high. When the engine oil pressure sensor resistance is low, the engine oil pressure is low. The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The IPC illuminates the check gages indicator when a low oil pressure condition exists. The engine oil pressure gage defaults to 0 kPa (0 psi) when the following condition occurs:
• The PCM detects a malfunction in the engine oil pressure sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
Fuel Gage
The IPC displays the fuel level on the analog fuel gage as determined by the PCM. The PCM monitors the signal circuit of the fuel level sensor. When the fuel level sensor resistance is high, the fuel gage indicates full. When the fuel level sensor resistance is low, the fuel gage indicates empty. The IPC receives a class 2 message from the PCM indicating the filtered fuel level percent. The PCM uses 2 filters for the fuel level percent. The slow filter is used during normal operation. The fast filter is used when the engine is just started or when the vehicle is in Park or Neutral. The fast filter allows the fuel gage to reach the current fuel level quickly. The fuel gage defaults to empty when the following conditions occur:
• The PCM detects a malfunction in the fuel level sensor circuit. Refer to Diagnostic System Check - Instrument Cluster .
• The IPC detects a loss of class 2 communications with the PCM.
Reviewing the fuel system description will further explain how the system functions. Refer to:
• Fuel System Description in Engine Controls - 4.3L
• Fuel System Description in Engine Controls - 4.8L, 5.3L, and 6.0L.

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.

Flush your car's transmission fluid , but don't do it yourself or at local shops take it to Toyota and have them replace fluid and filter . Reason why they use special pumps to extract all fluids properly . And actually replace it correctly other shops the only thing they will do is drop the oil pan and drain the fluid in the oil pan only . There is still fluid all over the lines and torque converter . Recommend dealer all the way , it's only 10-30 $. Different .besides you get genuine parts and fluids . Good luck I hope this help .

replace oil level sensor

for the powerstearing check the oil and refull when nessesary,turn engine on turn 5 times hole the way left and 5 times hole the way right.
check level oil again refull when needed.the white smoke on that spot
you say can leak from diesel.

They are designed to change as the motor changes speed. If you have 80 lbs. of oil pressure than you have very good oil pressure in your engine. If it gets higher than 80, I would talk to service dept. at your local dealer.

The oil light is an indicator of oil pressure, not oil level. If it is chiming all the time now, it means that your engine likely has an oiling issue, such as a failing oil pump.

Get the vehicle checked out immediately before you cause irreversible damage that requires a replacement engine or an entirely new car.

I have a 2000 Tacoma Extra Cab Pre Runner. When I put on my camper shell my check engine light comes on. Oil levels and tires are fine. I took my shell off and light goes off. I hauled alot of wood and the light did not come on. Why would the camper shell be trip-en my light? Ofcourse I am gunna get the code checked next week.

auto zone will read obd2 codes for free, to make sure this is fuel related.
when u get gas make sure the cap clicks at least 3 times when u tighten it.
replace gas cap.

Sound like there is too much oil in the vehicle, check the oil level with this vehicle on a level surface(if the engine was running prior to checking) let the car sit with the engine off for about 3-5 minutes before you check. If there is too much oil just drain enough to make it to the right level, this should take care of your problem. Good luck and thanks for using FIX YA