A 10 weight hydraulic oil will work fine. I personally have been using universal hydraulic transmission oil with good luck, but that's so I don't get cross contamination with my newer tractors. Any AGCO dealer can get you the filter, or visit <!-- w --><a class="postlink" href="http://www.agpartsfirst.com">www.agpartsfirst.com</a><!-- w --> for it. The Oliver part number for it is 103096AS. The Wix filter number for it is 33110. Any auto parts store worth their muster will be able to cross that number and get you a filter.

On your dash, the key is not original. There should be a large button for the starter, a pull switch for ignition, and a pull switch for lights. They didn't have a key switch until 1957.