You may want to look at Red Line MTL which is a very low viscosity synthetic gear-box oil that is safe for bronze parts.
http://www.redlineoil.com/products_gearlubricants.asp?productID=40&subCategoryID=15&categoryID=6
http://www.redlineoil.com/products_gearlubricants.asp?productID=40&subCategoryID=15&categoryID=6