Having seen several posts regarding recommended oil and fill guidelines, I found this document which is from one of the manufacturers of hubs, UFP.
UFP specifies 50W engine oil and recommends filling completely and not leaving any air at the top since air expands and contracts more than oil. Anyway, just wanted to pass it on.
Edit: This applies to UFP hubs only. Mine is a '02 and the axle has UFP stamped on the torsion arm. If you read on, it appears UFP was only used for a short time, and MC switched to Reliable.