gooser,
Sounds like you are asking about a King Quad 300 4x4. If so, I'll offer what I know.
1. Yes, the engine, tranny and rear differential all share the same oil. 10W-40 is recommended.
2. Oil filter is found in the right side of the engine case. Look low on the engine case in front of the rear tire for the sight glass to check oil level. The filter is located at about the 10 or 11 o'clock position in relation to this sight glass. There are three smaller bolts in a triangular pattern surrounding a larger bolt. The three small bolts hold the cover in place. Remove the small bolts and then remove the oil filter cover. Not necessary to remove the large bolt. Pay attention to the positioning of the filter when you remove the cover. It makes a difference which way you install the new filter.
3. I'm going by memory here, but I only recall one drain plug under the rear of the machine. I believe it takes a 14mm socket.
4. You are absolutely correct, gooser. That's where you add the oil. According to the manual, it should be filled until it is not quite to the top of the sight glass with the machine parked on a level surface. I have always just put an even 4 quarts of oil in it. The sight glass will now be full to the top, but this has never posed a problem for me.
Hope this has been of some help to you.
Shaylex