1. YOu don't want to use Redline MTL in your differential. MTL is a product made by Redline. YOu should be able to use any 80w90 gear oil in the diff.

2. I'm assuming a manual transmission which calls for a 75w90 weight gear oil which is GL-4 compliant. the GL-4 rating is very important as newer GL-5 compliant oils have more additives that can harm the soft metals in your tranny

3. There are several brands that people are using. Redline, royal purple, amsoil etc. I'm currently using redline mt-90 synthetic and have had no issues. You can use the same gear oil that you use in your transmission for your transfer case

Hope this helps

*edit* Shit just realized you said you had a 4 banger, I'm not sure if what i said above applies to 4 cylinder version or not. Someone else might chime in here