That is the classic valve stem seal leaking symptom. They are cheap ... be sure to get the later design with the gas sealing lip and install carefully. I would not do this job without replacing the springs... they are cheap and do get metal fatigue over the years... even just sitting with the engine not running some are going to be in the compressed position... they are cheap also given the labor needed to go back and replace them if you don't do it when you are already there... also consider putting in new valve rotators...cheap also...
The next step up... cam, bearings and towers... I would only do if they show to be past tolerances given in the FSM.