Oil wont have anything to do with the 'crunching' when downshifting to 2nd.
What model gearbox is it?
If it's a late 240Z or 260Z 71B unit, it'll be a Steel 'Bulk Ring' type syncromesh.
2nd & 3rd in these gearbox's cop the most abuse, and I'd say when the box has been rebuilt, they did not...