[Corey Young]

There is a round pate on the back with 3 bolts holding it on. The oil needs to be 1/3 of the way up that plate. That would make the oil level to the bottom of the idler gear roller bearings.

Stole this pic from ORD website

 

[Corey Young]

Search around for NP205 sight tube and there are a lot of different ways to do it. Pretty simple too. A couple push lock 90 degree fittings a piece of clear tube.

 

[Corey Young]

Here is a good article https://www.nc4x4.com/forum/threads/flat-gear-driven-t-case-oiling.153665/

Internet also says to make sure the clear tube has an operating temp of around 240 degrees. The oil might get that hot and can melt low quality clear tubing.