Pat,
There are (that I know of) copies of three slightly different (and different age) Sears owner's manuals around on the 101.07301. The two older ones say SAE 10 motor oil. The newer one says SAE 20 motor oil. I would go with the latter. Sometime in probably the 1950's, Atlas changed the recommended oil viscosity for all of the machines it built from SAE 10 to SAE 20. You should download what's in DOWNLOADS.
At the time that the 101.07301 first appeared, the differences were legs, bed (but only because some of the mounting holes are in different locations), and the parts in the headstock that were different only because of the sleeve bearings. The rest of the headstock assembly and all of the rest of the lathe except possibly the countershaft assembly were the same. In 1940, Atlas changed the countershaft on the 618 again to the B version. It isn't clear exactly what the the original countershaft on the 101.07301 was (whether it was the same as on the 1939 618 which was probably M6-20A) because all three of the known parts lists show the countershaft bracket as L9-20A.
