Actually when the specs were written, there was not a gear oil available with ep additives which did not have yelllow metal compatibility problems, that's why it called for either 50wt engine oil or 90 wt mineral oil (which is a gl-4 rated oil). Now however, there are several gear oils on the market which meet gl-5 and have a good ep additive for extra protection and also meet the gl-4 spec.
The product we recommend is the swepco 210 - 80w140 multigrade gear oil, which is available at the ihon store. You can use this product in the tranny as well as the tfc, and also in the differentials.
This subject has been covered in great detail earlier in the oil tech section.