Have a conversation with someone at your local farmers' co-op about what type of fluid is available. Since the dump system has only one function, raising the loaded bed, heavy fluid shouldn't present a problem. Does your system have a reserve tank, or does it use the upper side of the cylinder as its storage section? Some of the more primitive systems simply store enough fluid above the cylinder piston to supply the pump as it raises the bed, while others have a frame-mounted reserve tank onboard.
Jerry
