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.