2005 oil gauge going crazy

[franky69c10]

okay we changed the oil on my moms 2005 chevy tahoe and now the gauge is going crazy when you drive it stays at 80 and when you press on the gas it drops all the way to zero and would stay there till we slow down like at red light then it will go back up, sometimes it starts beeping saying oil pressure too low, the motor has fresh oil and the oil level is good can anyone help me out on this I think my dad use 10w30 can that cause the pressure switch to go bad
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