Any suggestions on fluid for a log splitter?
Generally any good heavy duty AW (antiwear) fluid, probalby in a "46" (ISO46) or "68" viscosity. Your system is positive displacement, that is the pump is always pumping fluid; you should let the oil warm to (OPINION) 90F at least before going to full pressure. Many people use ATF, not a bad choice really, but it, like most vehicle fluids captures and holds moisture; this is not a good characteristic for hydraulics. If you use it, change it every year or so.
The splitter came with a return filter only, I added a suction filter.
Beware suction FILTER; if it is a strainer, ok. Restrictions and air ingression in inlets may destroy more pumps than anything else. Do note that your system always runs fluid through the return filter when the pump is turning, a good thing. You might look at the return filter specs and improve them; a Beta rated element, B(10)>50, should be sufficient for you. Those 25 micron paper media things are JUNK in hydraulic applications. What brand and part number is your return element?