If you have the Toyo Koygo 5-speed then yes, use 80w gear oil
The Toyo Koygo will indeed have separate drain and fill plugs, M5OD only one fill plug.
And M5OD does use ATF Mercon
On drivers door sticker TRANS code will be "5" for Toyo Koygo
"M" for M5OD-R1
Example of door sticker here: http://www.therangerstation.com/tech...le_codes.shtml
At the bottom of sticker is TRANS code, and AXLE code if interested