SAE 40 has been hard to find for decades. It was specified for use in Midget/Sprite transmissions back in the 1960's, and was almost not anywhere on the shefl. No WEB to search either. I kind of remember getting it as Sears Robuck in their automotive section. That for sure will not work too...
GR:"The 17 hp has over revved and basically blew up! it will start and overrev and blow oil out the breather tube all over."
If it will start and run, then the internals have not blown up yet. For both the Briggs and the push mower, check that the governor linkage is still connected and moves...