Cheap 30w motor oil shouldn't hurt it and would be cheaper than hydraulic fluid. Older cylinders were run on motor oil (ie dump trucks).
I don't recommend it as a substitute, though I know it is used in certain (mostly older) applications, but it should be find for flushing the cylinder.
I am assuming you aren't running through your tractor, which would contaminate your pump, reservoir, etc. How do you plan to flush the cylinders?
Maybe someone else can chime in about the extra wear protection, and what not that hydraulic oil gives you over motor oil.
- JC