Any resolution to this? Shifting stayed OK?
I agree with BLS to take a close look at the clutch, maybe adjust it for a tiny bit more clearance, it might be right on the edge and dragging just a bit when hot (or cold, or something). Also if it's hydraulic, bleed it, if it's a cable, pressure lube it. It's a required maintenance service anyway.
As to not mixing oils, the reason I would recommend that is when debugging a problem, you want to change one variable at a time, see if that changes anything, then try something else. You can consider your mixed oil results as your first data point.