The op says he's in neutral, then can't get it back into gear.
If the trans is in neutral, and the clutch is engaged, everything is turning.
When you push the clutch pedal in, there is usually still some amount of drag helping to keep everything spinning, and when you go to put it into gear, the synchro has the job of stopping the rotation and aligning the teeth so it will slide into gear.
Surely you've heard someone grinding a gear to get it engaged........ They don't grind when they're not turning.
I still say a fluid change would help, and the Pennzoil synchro-mesh was recommended to me by a guy who does a lot of transmissions.
It's not that expensive and it's easy to do.
Thats true when in neutral and the clutch is out the input shaft is turning as is the countershaft and the syncro's do work when you shift into gear since the gears are turning. Try this and when you push in the clutch and go to put it in first wait about 10 seconds before you try to put it in first after you push the clutch all the way in. By then all the gears should have stopped turning and it should go right in first with no grinding. If it does not and seems no different then when you go into first as soon as you push the clutch in then something is causing the input shaft to still be spinning when the clutch is in. I have seen a pilot bearing that was to tight cause this years ago. If it goes into gear normally then you know the input shaft stopped turning as it should.
Myself I found that if its a clutch adjustment not letting the clutch disengage all the way you will usually also have shifting problems with all gears. I would still make sure the clutch adjustment is right but it does sound like the syncro is not working right. Can the wrong fluid be the cause ?? Myself I have not run into that but from what others have seen the wrong fluid may affect how the syncro's work so it surely wont hurt to change the fluid and try it. Good luck with it , Ron
