Here's the deal . . .
The Service Manual for the YSM 12 suggest 10W30 diesel grade (CP etc.) motor oil. I use puppy pads, or plastic backed absorbent pads to place under the engine and on the sole. The others are correct about having the engine warm. I pump the old oil into a 2 gallon plastic gas can marked "oil" in Magic Marker. I take it with me for disposal when I have the oil changed on the truck. Some waste recycling centers have oil disposal drops.The oil filter is located in the lower front adjacent to the dipstick. It has a spin handle and is held in place with three bolts. You are supposed to turn this handle a quarter turn each time you check the oil which is before each engine start up. The handle, plate and filter is removed as single unit. Observe the line up of the filter element in the engine. There is a notch that should be (as memory serves me) at the 2 o'clock position. The filter is spin type. By spinning the filter it clears the filter media which is metal. After removal, you can rinse the filter with a little diesel fuel. Dry it and continue to clean it in new clean oil. Use and old pie pan filled with oil and swoosh the filter through the oil to clean and saturate it. The only part that needs to be replaced is a gasket. If it is in good shape, forget it. Replacements are available at WM.As stated, the filler in the back is for the transmission. You pump it out separately from the engine. According to the Service Manual, use the same type oil as used for the engine.