If the Rods and shifter stops (if it has them) are adjusted correctly
w/brewers. If the synchros are worn, they will have a hard time getting through a slicker (synthetic) oil to grab the driven gear. Kinda like how you're not supposed to run synthetic when breaking in an engine because it will prevent the rings from seating.
The 2nd gear synchro is pretty common to wear out first.
Is this a stock shifter/rods?
Will it go in if you double clutch it?