As I said, it's a slippery and viscosity issue. Too slippery the synchros slip too much, not enough slippery and the other parts wear too fast. It's a critical balance.
Through the years the transmissions have undergone changes. What was perfect for one year may be terrible for the next year.
The cones have to slip just enough but not too much. For many many years they all used plain old 90 wt. Not so anymore. Using something that the trans was not designed around is taking your chances.
But then, the hard 2nd, 3rd entrance is not always just a fluid problem. Broken or worn parts can be the culprit - broken, worn brass rings (cones), bad synchro hub, springs or clips, even too much endplay in the mainshaft. All can be very expensive, mostly the labor.
Why would they go bad? Could it be from using the wrong stuff, making it harder to shift, leading to parts breakage?
What's that saying - you can lead a horse -----------------.