A lot of the dump trucks I've worked on used 30 weight non-detergent motor oil as fluid to operate the bed cylinder. If you're going to do any work on the truck with the bed raised, (even adding fluid to the reservoir) use a 4X4 post or something similar between the bed and the ground (NOT THE FRAME) to be sure the bed doesn't drop unexpectedly. Those things can be deadly, especially if a hose blows out, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!