Having a pressure filter (return line) is ALWAYS best, but in lower pressure, intermittent use applications like this, seldom used. More than likely your have a suction screen in the tank at the pump inlet, probably 100 microns. Most are, but down to 20 micron are available.
Easiest is just to use a fine a one as you can find and then use a few strong magnets to catch the nasty particles it doesn't.
With this it will easily outlive you with daily use.
Cleaning the system and tank with diesel or gas is adequate.
Just get the cheapest hyd oil (same as tranny fluid in this app) at the local farm store. Both just have anti foaming agents for this purpose.
You can check Donaldson or Baldwin spinon filters and housings if you want to get more creative. You might find deals used on farm or heavy equipment junk yards.
Late to the party with this tidbit, but inexpensively modding your cylinder piston to accept a polymer wear band will eliminate cylinder bore scoring.