Fuel pump runs after shut off- help needed!

[53GMC]

Just recently I noticed that after I turn off the engine the fuel pump used to stay on for 3-5 seconds but it started staying on longer.

Here's what I know - check if I'm right... The fuel pump receives its power from the fuel pump relay or the oil pressure switch (they are connected in parallel so that power can be provided from either source). The oil pressure switch provides power to the pump when the oil pressure reaches 4 PSI. The Fuel Pump relay supplies power to the pump when it receives a signal from the ignition switch. They are no other switches or interupts in the circuit



Couldn't I just unplug the oil sending unit and if the fuel pump doesn't run after I shut the truck off, then I will know it's the sending unit that needs to be replaced correct? I replaced the pump only a year ago and do not think it's that. I changed the oil yesterday. Could that have changed oil pressure reading any now?

Thanks for your help!!
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