Chev Express 2500 - it's a 2006 and in the City, oil pressure is nornal. As soon as I go on the highway, it drops to half of that. Can't figure this out.

Sounds like a week fuel pump have the pressure checked at a local repair shop. Also if it has a lot of miles on it the catalytic converter could be stopped up also have the local shop check it out it is real easy to check.

Have the oil pressure checked with a special test tool, you may have a defective oil pressure sending unit. If the pressure is low then you have other more serious problems and the truck should not be driven until the oil pressure issue is resolved, low pressure will destroy the engine if left unrepaired.

Could be the Oil Pressure Sending Unit. I do recall GMC having issues with the instrument cluster on 2003's. Try slapping the dash when the gage reads zero.

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.

If your oil pressure is running in the red this would indicate too much oil in the system. You should have your oil changed and refilled soon or you run the risk of burning out the oil pump. At idle your pressure should be just over a quarter of what your gauge has. At highway speed it should not exceed 3/4 of the gauge unless your going in excess of 100 MPH. Change the oil and filter and your vehicle should be fine.

Tune ups, clean oil & filters , tire air pressure at rated as specied on door panel. good driving habits.. have dealer ck computer settings

Most likely cause is a faulty oil pressure sending unit.

You have an oil leak. From where is the question. On most Dodge products, the oil pressure sensor is behind the oil filter. Petroleum products are solvents, however mild. This affects gaskets, wires, hoses, you name it. Have someone check the wires on the oil pressure sensor. Also see if it is screwed in tight. Neons are buzzy little engines and tend to have minor stuff wiggle loose from time to time. There is the possibility of having a "Double Gasket" on the oil filter which would cause the oil to leak and pressure to drop at lower RPMs. If all of that checks out, then you might need a new oil pump.

u will have to drop the oil pan or check on the back of the intake you since you are loosing oil pressure that would be more like the pump its self

code p 1870 refers to transmission component slipping
That can be from dirty oil or a clutch pack or band slipping or low oil pump pressure in the transmission
you say that it occurs on highway driving after 2 hours and that slipping will add friction heat to the transmission oil bringing on the oil temp light
take it to an accredited transmission professional service center that works on your make of vehicle , have the oil changed which require the pan removal
at the same time that will allow the adjustments to be done
when reassembled and refilled , have a pump pressure test done