I don't think that 6 or 7 pounds of oil located in the top frame rail would make much difference in a race bike either mate, many race bikes had much larger than "normal" fuel tanks mounted in the same location for endurance races, (I rode an Army version of Yamaha's TT600 with a stupid 40 litre "saddle tank" once, and that
was scary) but I think Fritz Egli, Tony Foale etc, saw some convenience in using the large "spine frame" top tube as an oil tank.
The Egli frames in particular were designed to accommodate just about any big Jap 4 cylinder engine of the era just by swapping engine mounts, so not having to design and manufacture a separate oil tank just for CB750's was a good way of keeping costs down. As it turned out, it wasn't the best of ideas as there were oil starvation problems inherent in the design, but they usually only manifested in racing conditions.
"Oil in the frame" designs weren't new even back then, most of the British Motorcycle manufacturers had built bikes with their oil tanks mounted in the frame at one time or another. Cheers, Terry.