Dunno about the B9A. Not a model I'm familiar with at all -- doesn't sound like a US-released Tohatsu model.
The (very old) M5A is similar to the M5B, though you may find the water pump is quite different. Parts catalog is "long out of print". Yes, the LU does use normal gear oil. Had one in the shop this past summer. Just needed a de-gunking of the fuel system.
I believe the M5A motors run at 50:1 gas/oil ratio, with the modern TCW-3 oils, or with a good synthetic 2-stroke oil, such as Amsoil Saber. I have no idea of that oil ratio for the B9A (sounds like a Honda model).
Sorry, I know that's not much of an answer. Tohatsu Guru may have more historic information.
The B9A ia an old 5 hp back from the early 70's. There are no parts or manuals available for it. The M5A is from the late70's to early 80's....Same story. I don't think the B9A was imported into the U.S. and there were very few of the 5A brought in.