It's not a gear oil issue. It's your clutch pedal assembly or a hydraulic issue because it's inconsistent. I bet if you went to Auto Zone today, bought a master and slave cylinder, swapped 'em out and bled the clutch, this would go away completely.

You never answered whether or not there was anything leaking from the clutch master inside the car. Push your finger on the rubber boot and see if anything leaks out of it.