I am currently experiencing an oil pressure issue on my 07 LH6 Sierra. I have 138,500 miles on the motor and on cold starts 8 hours or so or on very cold days the gauge takes a second or 2 to pressurize up to about 50 psi. Once I begin driving it while it's cold or idling to warm up, the pressure will drop to 15-20psi at stop lights but is at 50psi driving, 2 or 3 times the lifters have made noise because of it, but don't make noise while the pressure is maintained. I make sure to keep it above 20psi at stop lights and such just so nothing terrible occurs. After the motor is at operating temp 210*, the pressure maintains 40psi at idle and 60psi while driving and is as quiet as a church mouse. I've run Mobil 1 5w30 since 60k when I bought it except for 2 or 3 times it had Dino in it. The pressure sensor was replaced last winter with a Napa one and is functioning properly according to a tech 2 scanner. I spoke to the mechanic who maintains the ambulances for the dept I'm in and he said he believes the motor is shot and should trade it in. I don't want to believe that, any insight? Is it a dead motor? Is it the o ring on the pick up tube sucking air until the metals warm and seal properly? I'd like to keep this truck a few more years since it's paid off.