P300, cylinder 4

[Gtir007]

15 denali, misfire in cylinder 4, changed plug,switched coil, switched wire, same result. I am suspecting it's a bad lifter, but it's quiet, no ticking. Towed it to the dealer, they want to charge me $600 to remove the valve cover and 895 to tear down the motor. At this point I'm going back for my truck and fix it myself. Truck has 77k. I also noticed that my oil was very low. When I start it i,t would smoke and then would disappear. Can a failed afm cause major oil loss. I'm not about to pay the dealer thousands of dollars, parts cost nothing.Any input would be appreciated , thank you