I have a 12 ton(?) MTD log splitter that I bought used last year. I know
how to maintain small engines, so the 5HP gas motor for the hydraulic pump
is easy.

The splitter, since it was bought used from an individual, didn't have any
manuals.

My question is about the actual hydraulics. Do I ever have to change the
fluid? How often should I change the filter on the hydraulic system? I
know I need to make sure there is enough fluid in the system, but what kind
is best?