When it comes to antique iron.. keep in mind that -any- oil you can buy right now.. will be superior to what that thing probably ran on new.. and that includes brigs and stratton 30w non-det, and 'jiffy-store' 84 cents a quart type stuff.
On my B I changed the oil about 3 times the first week I had it to get it completely flushed out. i used supertech oil from wallmart.
Soundguy