Oilpump Only down a quart of oil and oil pressure guage only down slightly. Check engine light flashing. Stick shift and engine seem to be shaking upon acceleration. Could this be oil pump?

Most engines take at least 5 quarts of oil. Have you check the dip stick to see how much is in there?

next time you change the oil use a quart of Lucus oil treatment inplace of one quart of oil. The oil is warmed up it loses viscosity so that is normal to have lower oil pressure. I use Lucus oil treatment along with Mobil 1 and my truck runs great with just a little higher oil pressure this makes me feel all warm and fuzzy inside.

it could be as simple as a bad oil pressure, or it could be a bad engine. If the oil pressure was good before the oil change maybe the oil filter might be the problem

it can be that your oilpump is packing up or your oil switch is faulty.the best bet is to do a oil presure test to check oilpump then you can work from there

When your car is low on oil, there is a sensor (sometimes called a switch, as well) that will light up an indicator inside the car to inform you of the low oil level. In some cases, checking the dipstick will show that there is actually oil present, which can be confusing and frustrating because it contradicts the sensor. If this happens to be the case, more than likely the oil pressure switch/sensor is malfunctioning and will need replacing.

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if the oil light is flashing but there is oil in the engine then it is probably the oil pressure that is the problem, you need to attatch a oil pressure guage and find out what the pressure needs to be

the oil sending unit is bad

get oil pressure test done and check for codes in PCM

It could be the oil pressure sensor for the oil light or guage.I have seen this before after age and heat plus vibration could cause it to crack and leak a lot of oil like you describe.Get it fixed asap.It should be easy to unscrew with a wrench or large socket .Buy the new sensor first though so you can replace it once you get the old one out.It leaks alot because of oil pressure from the oilpump pressurizes the rest of the engine bearings.It could break altogether and pump all your oil out and ruin your engine.

The sending unit for the oil guage will cause the oil guage to fluctuate this is caused from some sludge build up that is more than likely stuck in the sending unit tube. Use Gunk motor flush on your next oil change this should help the oil guage problem.( make sure you read the direction on the back of the bottle)
As for the check engine light you can take it to Auto Zone and have them scan the computer. Sometimes the EGR valve will build up with carbon causing the valve to stick slightly open which will make the idle on the car fluctuate up and down. Its also possible you need O2 sensors.