Yardman HE5150 Hydro workshop manual?

Does anyone know where I can get a workshop manual for my Yardman HE5150 Hydro. I service it myself and have greased every grease nipple I can see, changed engine oil, cleanded spark plug and filter but I don't know how to change the greabox oil(there doesn't appear to be a drain plug) and what oil to replace it with.
Any help ,many thanks