My transmission used to do that all the time - very annoying - until I discovered I had way too much clutch play - my pedal was almost to the floor before it even started deflecting the clutch. Adjusted that up to 1" play and no more grinding - except going from 3rd to 2nd of course - always have to double clutch that one.
Here's how to adjust the clutch free play:
Also, you mentioned something about using the original 600W oil. There warn't no such thing. Here is the spec for the oil to put in the transmission - S.A.E 90 summer, 80 winter. I use 85W-90 year round:
