I'm going to do the same thing this weekend so I just looked in the service manual. I have a 46 1/2 ton and below is what was written in the original book.

"While seasonal changes of the lubricant are not required, it is recommended that the housing be drained and refilled with the recommended lubricant at least twice a year, or every 6,000 - 10,000 miles."

For the lubricant it came from the factory the manual says...

"1/2 ton Hypoid rear axle - SAE 90"

Hope that helps

Mike

My blog and photos of the old 46 --> http://kens46truck.wordpress.com/