Oil gauge drops low at idle

Sounds more like an engine idle issue than a oil pressure issue. If the engine idle drops too low, the oil pump cannot provide sufficient oil pressure. If there is no oil light when the engine is idling normally, then there is no problem there.

Do not drive the truck until the issue is resolved.

1. Make sure the oil is at the proper level in the engine with the correct spec oil for the pickup. Oh it is normal for the oil pressure to be a little low at idle and go up to the normal range as the engine rpm goes up to normal operating range.

2. It could be either a real problem or a problem with the oil pressure indicator. To verify which connect a mechanical test pressure gauge of the proper range say a 0-100 psig scale to the point where the oil pressure sensor is connected. After it is connected test run the engine for a short period. If the pressure shows normal replace the oil pressure sensor. If the gauge confirms the low oil pressure then you have a serious issue in the engine. It could be:

1. Bad oil pump or oil pressure relief valve

2. excessive clearance in the bearings due to high miles.

Well according to the owners manual, anything from low up to the last mark is normal. I think they mean anything from the low mark up half way is normal for idle.
If the gauge stays near L at idle and while driving, you could have low oil pressure or a gauge problem. A mechanic would check pressure with a mechanical gauge.
Normal for most engines is around 25-40 psi at idle and 30-60 psi when driving.

this means a mechanical gauge test is required to check actual pressure warm idling if low below 20 psi then the oil pressure pump relief is stuck open or engine bearing are worn

see this tips and fix it. God bless you
The oil pump supplies oil to lubricate your engine. If the oil pump is worn or is not turning, the engine will suffer a loss of oil pressure, which may result in engine damage or engine failure.
The first sign of trouble may be a low oil pressure warning light, a drop in the normal reading on you oil pressure gauge (if your car has one), or the appearance of ticking or clattering sounds from your engine.
As a rule, most engines only need about 10 PSI of oil pressure for every 1,000 RPM of engine speed. Oil pressure will read higher than normal when a cold engine is first started because the oil is thick. Oil pressure will gradually drop as the engine warms up and the oil thins out. So normal oil pressure on a warm engine cruising down the highway is typically 30 PSI up to 45 PSI.
SYMPTOMS OF OIL PUMP TROUBLE
The first thing you should do if any of these symptoms occur is to stop your car, turn off the engine, let it sit for a few minutes, then check the oil level on the dipstick. If the oil level is at or below the ADD line, add a quart of oil to bring the level back up to the full mark. Add as much oil as is needed to raise the level to the full mark. Then restart the engine. If the warning light remains on, or the oil pressure reading does not climb back up to its normal range, or the engine noise does not go away, you may have a bad oil pump.
The other possibilities include a bad oil pressure sending unit, or a problem with the oil pressure warning light circuit or oil pressure gauge.
OIL PRESSURE SENDING UNIT
If the engine is NOT making any unusual noises and seems to be running normally, and the oil level on the dipstick is FULL, but you are still getting a low oil pressure warning light or low gauge reading, the fault could be a bad oil pressure sending unit.
The oil pressure sending unit is mounted on the engine block. On some applications, there is a spring-loaded pressure-sensitive diaphragm with a switch inside the sending unit. This switch completes the circuit to the low oil pressure warning light if oil pressure drops below a certain threshold. The unit may stop working if the diaphragm inside fails, if the switch is stuck, if the small hole that allows oil to enter the sending unit becomes plugged, if there is a loose, corroded or broken wiring connector at the sending unit, or there is a fault in the wiring circuit between the sending unit and warming light.
On vehicles that have an oil pressure gauge (electronic, not mechanical), the oil pressure sending unit has a small rheostat inside that sends a variable voltage signal to the oil pressure gauge when the diaphragm moves. A worn spot on the rheostat or any of the other problems just described for the simple pressure-type oil pressure switches can cause a problem.
FORD'S FAKE OIL PRESSURE GAUGE
On many Ford vehicles that were built from 1980 through the 1990s, the oil pressure sending unit has two switches, a low pressure and a high pressure. These vehicles also have an oil pressure gauge, but the reading on the gauge is not a true indication of real oil pressure. As long as the pressure to the sending unit is between high and low, the gauge will read normal. If oil pressure drops and trips the low pressure switch, the dash gauge will now read low. Or, if oil pressure goes up and trips the high switch inside the sending unit, the dash gauge will read high. Consequently, don't rely on the oil pressure gauge for an accurate reading in these vehicles. It is only a gross indication if the oil pressure is low, normal or high.
OIL GAUGE PROBLEMS
If the engine is NOT making any unusual noises and seems to be running normally, the oil level on the dipstick is FULL, and you have replaced the oil pressure sending unit but are still getting a low oil pressure reading on the dash gauge, the fault could be in the wiring circuit between the sending unit and gauge, or the gauge itself might be bad.
Check the wiring connections on both ends as well as wiring continuity between the sending unit and gauge. If no wiring faults are found, hook up a pressure gauge directly to the oil pressure port on the engine and check oil pressure with the engine running. If the engine-mounted gauge shows normal oil pressure but the dash gauge is reading low, the problem is a bad dash gauge.
On the other hand, if the engine-mounted pressure gauge reads low and you have done all of the above, chances are the oil pump is worn, or it is not picking up enough oil because of a restriction or blockage in the pickup screen in the bottom of the crankcase.
OIL PUMP PICKUP PROBLEMS
The pickup tube has a screen on the end to prevent large chunks of anything bad that ends up in the crankcase from being sucked into the pump. But we are talking BIG chunks of debris, not normal wear particles or carbon or dust or other microscopic-sized abrasive particles that can cause pump wear over time.

The engine fan is controlled by a relay and a radiator temperature switch.
Unplug the switch at the radiator, and if the fan still runs, the relay is stuck on.
If you unplug the relay, it should stop. If not, you did not pull correct relay.

Engine oil gauge is for pressure, not level. And pressure should always drop at idle speeds. As long as it is over 40 psi when at higher speeds, then oil should be ok. Normally at idle, the pressure will drop as low as 7 psi.

how did you clean the gasket off of the head? if you used a sanding disc the grit will eat up the bearings. are you reading pressure with a mechanical gauge? you have replaced all bearings??

6 psi @ 1000 RPM
18 psi @ 2000 RPM
24 psi @ 4000 RPM
this is chevrolet specs on oil pressure in your truck so you are within the range.

describe your idle problem more and i will try and help you with that

You could have three possable problems that I can think of. 1) Your oil viscosity is too low and you need to change it to a higher level. A 5w/30 or 10w/30 oil may thin out too much when it warms up to normal temp and shows low on your gauge. 2) The preasure relief valve is partially open due to a small piece of debris caught under the seat. Unscrew the oil filter and look at the small disk that is near the threded part on the filter base. Theres a spring behind it to keep it closed so all the oil goes into the engine and not bypass back into the oil pan. Push up on it slightly with a small tip screw driver and see that it fully seats in the hole, put a new filter on, and try it. 3) If these two things don't improve the condition, get a socket for taking the oil preasure sender off and get a oil preasure gauge to test the actual oil preasure the engine is producing. You may have a weak pump in the engine. If the preasure is within good specs, I'd change the sender unit with a new one.
Let me know what happened-
Craig

These cars are funky and this is normal...gauges on all of the earlier cars are for general reference...checking the oil level manually while the car is running and warmed up is the most proper way to do this...generally speaking, your pressure should drop as the car warms up, but still reach 5 bar on full accelleration (928's have the same gauge). The oil level gauge will actually increase as the car warms up...the difference can be dramatic.

check coolant level ,must be low ,chck for coolant loss