I am presenting this for my knowledge and maybe to help other Nissan owners, especially since unaware owners can get fleeced in these hard-sale days. For 1993 Altima with 180k miles, does the valve lash normally need adjustment? (DOHC w/o hydraulic spacers). I asked this at a Nissan service desk and they looked at each other and said they had never done it as routine maintenance; Only if a problem existed (very loud valves). I asked the question due to apparent blow-back when oil filler cap is removed; but seems this is unlikely valve lash caused. And no, I have not done a compressions test. Engine starts and runs great; plugs are perfect, visual look into combustion chambers all clean!

No, there is no maintenance interval for the valve train on that car.

A compression test might be in order, but you'll learn far more from a leak-down test.
There may be nothing wrong with your car's engien; many crankcases are referred to as "pressurized." By pressurizing the crankcase, even slightly, there is less chance of debris being introduce into the engine oil, improving oil quality and life. Some cars PCV are now threaded into place rather that slip-fitted due to pressurization.
If you're not experiencing excessive oil comsumption and oil isn't present in the air intake (near the PCV inlet hose) I would just drive the car and enjoy the scenery.

To anyone who will ask and then listen:
If you aren't sure what your service consultant is telling you, and he can't explain it to the point that you do, ask nicely for your car back and seek another opinion elsewhere.
Or, ask us here, if time allows you to.