Suction strainers and return filters serve different functions:
-Suction strainers (not filters) cannot restrict flow more than a psi or two or they will cavitate a pump and cause damage. So they have to be either really big, really coarse, or both. Their role is to remove large visible size particles, typically rust or slag, that can cause catastrophic damage in one pass through a valve or pump or motor. They will typically be 100 mesh finest, more often 20 or 40 mesh size. That is a couple hundred microns or more. It's a balancing act, which is more likely to cause problems, the dirt or the chance of cavitation. Gear pumps are more tolerable of cavitation than piston pumps where higher case pressure can destroy pump pretty fast
-Filters (almost always in return, rarely in high pressure locations) are to clean up the system and remove 10micron sized stuff, way below visible particle sizes. These particles are often generated in the loop from pump or motor wear, or ingested through pump motor or cylinder shaft seals. They won't cause catastrophic failure, nor even short term failure, but long term wear. By stopping the cycle of wear (particles wear iron and cause more particles which cause more wear, etc) they are for long term system protection.
They can have 20 to 40 psi pressure drop without causing problems, so they can be MUCH smaller than suction strainers.
Yes, many will say never needed filters, etc etc. And for a logsplitter, a few hours per year, they may not be needed. Usually the system condition shows it, but they will still work fine for years because of low hours.
I try to avoid suction strainers, or upsize them 2 to 4 times, or use 20 mesh or there about. Most of my designs (industrial and mobile equipment in REALLY dirty abusive environments) used sealed tanks with pressure/vac 10 micron breathers and bladders to prevent air going in and out.
For a splitter, I would use a fine mesh filler strainer basket (where you pour oil in) with good breather (not the steel wool type that basically just keep out objects the size of small animals...), NO suction strainer, and a good 10 micron return filter upsized. The typical splitter return filter is 20 to 40 microns, not very good.
Most heads are either the standard SAE 1 inch thread post, and many elements will interchange, or the standard metric thread, and also common elements. I don't have the napa numberts here but they are easily obtained, reasonably priced, and decent quality.