There is no service manual as such.
There are 2 ways to do an oil change ;-
1) Raise the mower so you can access the underside. DISCONNECT THE SPARK PLUG WIRE. Locate the drain plug - depending on the engine type, it will be a 3/8" socket head plug (a 3/8" drive socket extenson will fit), OR it will be a 3/8" MALE plug requiring a 3/8" open ended spanner to remove. It's quite obvious when you clean off all the crud on the bottom.
2) Remove the oil filler and physically tip the mower to empty it. Easier, but messier.
Refil with SAE30 oil.
phil_saunders(a)bigfoot.com
Phil Saunders
September 2009