There is a write up on 4x4wire about adding a sight to a Dana 300 T-case. I figured I'd always add one to my Dana 20.... (Can't find link right now, but it has some good info on the type of tube to use, and connectors)
Then when doing the stock pan for a temp gauge, I figure I'd try a sight tube on the C4. So far so good. I'll take pics tomorrow.
I just hooked the pan to the vent on the top, with a T to the firewall to breath.
It worked great when the last time I changed fluid, could see it being sucked in.
Note: I've a big flat belly pan that protects the tube.