Smart move to change the fluid. But read up on which fluid. It is almost as bad as the IMSB story!!
Great that it does not pop out of second. It will -unless you install a gizmo to prevent it. And once it starts there is no solution -just a very expensive rebuild. Ask me how I know...I now have 2 gearboxes - both in bits!
The Pop out detent is well known so search for more info on this Forum.
There are two sellers I know of ,Gbox and Raceline. Eric at Raceline often comments here to help out Forum members.
https://www.facebook.com/theraceline77
But please note this caveat from GBox:
"The upgraded detent will not fix a pop out problem if there is already internal damage to the gearbox, but it may certainly help prevent it. We install it in all of our rebuilds, and we have yet to see a 'pop-out' problem return after the installation of this part"
I hope this helps you.