So there is a lot of debate about fluid in the ax15. It seems gl5 is bad for the brass in the trans. If you or previous owner has run gl5 then that will cause premature wear on the brass in the trans. Although that said you have 157K on the trans, so that could just be wear.
What happens when you shift up or down in your AX15, and all transmissions, really is your clutch disengages your trans from the engine meaning with the clutch in, the transmission stops moving. Well there is a tiny bit of inertia. So the gears have synchros and what that does is overcome the left over inertia. They stop the gear completely so you can finish the shift. If you are grinding up or down it is probably a synchro that is wearing out, and 2nd is usually the first to go. You will probably notice 5th starting to get grindy next. It is probably time to go into the transmission. The reason why it works if you wait, is it is taking longer for the gear to actually stop moving.
And to even get precise in my explanation, Your not only getting a small amount of inertia from the front, but you are getting it from the rear via the rear wheels. So your synchros are overcoming inertia both ways.
I hope this helps
