I'm real familiar with that valvetrain, having raced smallblock chevys for many years and the 4cyl uses many interchangable pieces typical of GM. It sounds like the cam lobe is worn on that valve. they usually wear right through the bottom of the lifter when they fail, causing the oil to drain into the crankcase instead of coming up through the pushrod.
A simple way to check it is to remove the rocker from the bad valve, and one from an another cylinder (if the bad one an intake valve, make sure the other one is and vice versa) turn the engine over with a breaker bar and compare travel at the pushrod - a dial indicator in nice, but a machinists rule and good eye will do. If the travel is significantly different, you'll need a cam and lifters. You can confirm that by running a magnet through the used oil, if it picks up metal, you know.
If the travel is ok, check for a bent pushrod, rocker arm locknut tension and bent valve. if the pushrod is bent, find out why. Once in a great while we see the locknut loose tension and back off, you'll feel that right away when you turn it - compare to another. I hope that's it, because it's about the only non-serious cause. A new nut and a valve lash will fix that. Lastly, don't be tempted to just adjust the rocker down as it will just cause more damage and throw metal in the oil.
Hope this helps.
