Hmmmmm. If it's grinding a bit, sounds like more than dirty oil. How many miles on it? I would suggest an oil change, but check the oil coming out very carefully for any bits, especially in the plug. First thought is a missing tooth or 2 on the synchro hub or a broken detent spring in a synchro hub.
Can you be a bit more descriptive? Changing up / down feel, any other noises? Has the change been sudden?