For the past week, my wife's 2012 Tahoe with 94k oil pressure has been very up and down. At 2500 rpm or so, it would run close to 40 psi but drop back to about 30 psi. At idle, about 18 psi. Running between 1000 and 1500, it would level out at about 20/25 psi. No lifter noise, knocking, etc...