You need to take the body off first pins on bottom of either side and two pins in the fron hood come out, 4 pins (I think I only use 2) in the rear. Pull the bottoms out and up. May have to pop the front and rear up first.
Battery box is held on by 2 pins. There are tabs in the back of the box that slide into slots to keep it in place l. Remove pins and lift the lid off at an angle till the top comes off. Use what ever oil you want. Synthetic is better then dino oil. Not trying to go into a debate again about oil. If it's good for an air cooled 2 stroke use it.
Eventually you'll want to go lipo for the reciever but that can wait. If hump packs and small lips are all you'll be charging then there are a lot of chargers out there to fit the bill. Most the chargers I would recommend will be over kill for your needs. Someone else will chime in.
Ideally you'll take the whole thing apart and check it out. Diffs to make sure they are full and tight. And the cv/ drive shafts to loctite the drive pins/set screws.