I used Royal Purple Synchromax.
Many will say RedLine MT-90.
1- Remove 'normal' filling bolt driver side, mid hight. It is also used as level indicator.
2- Remove drain plug on the bottom let it drain.
3- Reinstall drain plug
4- Fill until spilling from side plug
5- Reinstall side plug.
... next will be the diff?
Jocelyn, you need to be careful with synchromax, it is NOT 75w-90, or even close to it. It is made more for manual transmissions that require atf fluid as their oil. In fact, Summit lists it as 10 weight!
In Royal
Purple's spec sheet, it clearly states this:
Royal Purple® Synchromax® is recommended for manual transmissions that specify an automatic transmission fluid or other light-viscosity
oil. It is also ideal for transfer cases and 2-cycle motorcycle gear boxes that specify light viscosity lubricants
While the W58 trans will work with it, it does wear the synchros down, as evident with the amount of silver I had in my truck's W56 trans when I replaced it many years ago.