My oil pressure gauge drops to zero when the vehicle slows. oil level is full. had oil changed recently and it was thick so they added a solution to keep that from happening. the gauge reads fine when the car is moving; most of the time it is dead center.

check the oil level in the sump to ensure that oil was added after draining it for the change
There is a oil pressure sensor / oil level gauge ( different to the oil pressure gauge sender unit) that will not allow the engine to start if there is no oil pressure sensed or no oil level sensed
check for wiring connections as well

Check your oil level

first suggestion is to check the oil level in the sump as this indicates that on start up there is sufficient oil but when the oil galleys are full the pump is sucking air and the pressure drops to zero. The level has to be on the top most fill mark.

check the oil pressure first, it should read around 10 to 14 psi. If pressure is inside the range then your pressure sensor is bad. If lower or reads 0 psi your got a engine problem, further diagnosis needed. Good luck.

oil pressure sensor, low pressure due to dying pump. or blown gasket causing the pressure to leak out. Check the easy ones first like the sensor. Good luck

Could be the gauge on the dash. If you really had no oil pressure the engine would be very loud.
A mechanic would check oil pressure with a mechanical gauge to verify.

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.

Have a mechanical gauge hooked up to see that the oil pressure is good or not. If you have no or low at an idle, it can be due to bad oil pump, or worn out bearings.

The guage may rise a little when you accelerate the vehicle...but if it is pegged high, and does not fluctuate, when you turn your ignition key on an off...Your oil pressure switch maybe going bad...Goodluck

Your oil becomes too fluid as the engine gets warmer. When the engine rotates slower at stops, the oil pump won't make pressure. At speeds, more oil is pumped and the pressure rises.
Have you recently changed your oil? Or maybe is too old. The real problem would be if you read zero at normal cruising revolutions.