For the sake of accuracy the rear end gearing in the Fords discussed here are classified as spiral bevel gears. To save me a bunch of keyboarding here is a pretty good description for those interested: http://en.wikipedia.org/wiki/Spiral_bevel_gear
As has been stated maybe a bazillion times on here, 600w in today's viscosity parlance is an ISO 460 grade, which converts directly to an SAE gear lube viscosity grade 140. Within a single companies products it could be very likely that an ISO 460 industrial lube and an SAE 140 automotive gear lube use the same base oil (which would be in excess of 90% of what's in the container), the only difference might be in the additive package (depending on intended application). Any perception of difference in viscosity or "feel" would indicate the need to get the eye, or fingers, or butt recalibrated.
I've been repeating the correct info for 15 years! The original 600w was ISO 680 with the nearest SAE gear oil equivalent being SAE 250. Ford changed to SAE 250 in 1932.
Ah good, someone who sweats the details. For the very few who care we might be able to advance the discussion.
I suspect, even in 1932, there were ambient temperature considerations specified. SAE 250, in relative terms for automotive use, is very viscous. In my opinion too viscous for all but the most extreme of high temps. Just as an example; heavy duty trucks and off road equipment that use rear end gears not particularly different in design and assembly specifics don't specify that viscous a lubricant today, and they are subject to much more load than any early Ford. Granted, there are differences in additive technology from 80ish years ago, but then why would we not upgrade with the times/technology?
As an aside, an ISO 680 is borderline for the measurement range specified for both SAE 140 and 250, so, depending on actual testing results it could be rated as one or the other.