There was a real nice article in Skinned Knuckles last year (October 2011, vol. 36 - no. 3, issue 423) on what is the correct gear oil for your old car. While the actual viscosity/weight of what gear lube should be used was not really discussed, one of the main points that was discussed was the gear lube containing sulfur and phosphorous and their detrimental effects on "yellow metal"meaning brass and bronze.
They had talked about the differnt ratings for gear lube (GL-1 through GL-5) and which one should be used if they are going into a rear axle or transission that contained any brass/bronze bushings. The gear lubes that were listed as being available were the GL-1, GL-4 and GL-5 (GL-2 and GL-3 were listed as being obsolete). If there are any of the "yellow metal" bushings in the axle or transmission both the GL-4 and GL-5 gear lubes should be avoided and only the GL-1 gear lube should be used. They went on to say that the detrimental effects/wear were less if the bushings in question were bronze instead of brass, but damage would still occur. Some of the new GL-5 gear lube does not contain the "active sulfur" which is detrimental to the "yellow metal" and the manufacturers have started to state that on their packaging as copper/brass safe.
Gear lubes and their content of sulfur and phosphorous had been discussed at some length on the forum of the MTFCA. This being more of an issue with the model T rear axle and their use of bronze thrust washers as a replacement for the original babbitt thrust washers. The only place that this would be of a concern with the model A is in the transmission due to the use of bronze bushings for the reverse idler gear and the bronze thrust washer for the earlier 1928 transmission cases. So that brinngs up the question about the 600W that the vendors are selling; What is it's rating GL-1, GL-4 or GL-5? And is the 600W sold by the vendors safe for use with the previously mentioned yellow metal?