I'd guess you know as much about these cylinders now as anyone you will find on here.
I think the name of the oil is vacuum cylinder oil, or booster cylinder oil, though when I had to service my DUKW hydrovac I used light jack oil (for a hydraulic jack) It has to be quite light and not attack the rubber seals, which a lot of ordinary oils will.
This sort of stuff...
http://cgi.ebay.com/Hydraulic-Jack-O...3286.m20.l1116