I have been using Aamsoil synthetic in my VX since 66K.
No changes or flush since then. Now I'm at 185K.
In otherwords do as little as possible.
You can check out your current fluid by opening the upper drain oil a crack and dripping some fluid out.
If it stains your fingers or smells burned, change it, if not, leave it alone. That's my two cents. Our transmission is identical to the Post Office LLV and they live up to their name.....Long Life Vehicle.