I think you may be confusing transmission/rearend oil with Oliver 55 hydraulic fluid. Any oil with the Oliver 55 spec will work in the HYDRAULICS. 80w-90 is what you want in the rear end.
Try this link:
<!-- m --><a class="postlink" href="http://olivergang.org/discus/messages/257/313.html">http://olivergang.org/discus/messages/257/313.html</a><!-- m -->