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Looks like a number of manuals for sale on eBay for it, did you look there? You might get lucky, but don't count on finding free ones. Here is a link to Hitachi's site to order manuals:https://www.hitachiconstruction.com/service-support/order-manuals/
For now to ID the tanks there are several things you can do. 1. Diesel should smell different than hydraulic. 2. Chase the hoses/lines from the tanks and see where they go (or work backwards from the hydraulic and injection pumps to the tanks). 3. Use clean sticks and "stick" the tanks to see what comes out on the stick, and how much is in the tank. 4. Or use a small hose/tube to pull a sample from each tank. You might have to remove screens in the filler necks for 3 and 4.