There are several slightly different versions of the 'Classic' B&S. Either there will be a dipstick with a cap on it and an oil can icon on the cap, which will be obvious, or there will be a screw-in plug on one of the rear corners of the crank case casting. THe plug will have 2 vertical prongs/lugs on it to enable a lever of some sort to be used to unscrew it.
If the plug is at the base of the engine, fill to the top of the hole.
If the plug is at the top of the casing (at the same level as the blower housing), there will be a dipstick attached to the plug.
phil_saunders@bigfoot.com
Phil Saunders, November 2008