My vote goes to a bad syncro. If fluid was the problem it would likely be an issue with all gears, not just second !! Clutch ... shouldn't be an issue if you have about an inch of freeplay in the pedal.

Ideally the syncro teeth should all be sharp - both the steel and the brass. However, I've had a few trannys with bad steel teeth and they've all worked fine once new brass rings were installed.

The problem with replacing the "steel" parts is the extent and expense. Its one thing to replace a slider/ring assembly but they're only half the equation ... the other half is the syncro teeth on each gear! If they're worn on the slider then the corresponding teeth on the gear should be shot too! Start replacing all the steel parts that have worn teeth and you may as well buy a new trans!

Pull the side cover, drop the driveshaft, yank the tailstock and replace all the brass rings, reassemble and drive it for years.