Whatever oil you buy, it must be rated GL-5. Do not use GL-4. The -5 has a much higher extreme pressure rating, and in a heavy Bus, you need that.
Any of the oils you find will be ok. Use 80W-90 if you pick ordinary oil. Synthetics come in 75W-90 for better cold weather shifting. I have ELF synthetic in my car right now, it's 70W-80 and in the winter it shifts just like summer.
You can install the axle boots with the Bus up in the air for easy access, but leave the small dia hose clamp loose until after you put it back on the ground. If you tighten that clamp up in the air, it will leak.