PTSG, what did you do with the rock?
Oh, on the hydraulic oil the answer is that since there aren't any accepted industry standards for hydraulic oil formulation, performance, and supplements in the US....and without any ANSI or SAE specs to go on either, then nobody can tell you for sure that hydraulic oils are compatible.
In the absence of problems it seems that operators just sort of assume that fluids can mix OK. I haven't heard of any compatiability failures from mixing hydraulic oils here or on other foums. And each time SSQA equipment is rented at skid steer rental yards it puts quarts of unknown fluid into expensive industrial hydraulics - but nobody seems worried there.
So the common evidence is OK.
.