Just curious,,, I have a small Kubota made in 1980. No hydraulic filter other than the suction screen that I clean when I change the hydraulic oil every 3 or 4 years. 2400 hrs and never a problem and still no leaks. Am I just lucky?
Early gear-drive compact tractors mostly did not have pressure-return filtration, as far as I know. Many/most of these old machines had low-pressure implement pumps to run the 3-point lift cylinder and a loader. Pressure was often limited to 1200 psi or even less, until the 70s/80s, when pressures went up to as high as 2000psi.
Also in the 80s came wet brakes, which shared oil with the whole hyd system. Then there was HST, also becoming more available in the 80s, with much higher pressure and sensitivity to dirt.
A tale of 3 tractors I have owned and repaired: (this IS a tractor site)
1) mid/late 80s Kubota B8200 4x4 gear (dry brakes, suction filter only, 1700psi pump)
2) mid/late 80s Kubota B20TLB 4x4 HST (dry brakes, pressure-return filtration, PS, 2000psi implement pumps)
3) late 80s Kubota L2550DT 4x4 gear (wet brakes, pressure-return filtration, PS, 2000+psi pump)
Nowadays, compact tractors pretty much "all" have wet brakes, and even the gear machines run pressure-return filtration and pump pressures of 2000-2500psi for the most part. I am sure I am going to hear about exceptions.
Even relatively cheap modern hyd machines out there today (e.g. log splitters) use return filtration and run pressures of 2000psi or more.
Low pressure non-tractor systems (under 1500psi) out there today commonly do not use return filtration. They also do not have continuous-duty like a hyd motor. e.g. power steering in your car. The Nissan PS pump I use for my dump bed is only 1100psi, and has nothing more than an inlet screen. Contaminates are controlled by not sharing the system with other high-duty hyd motors and wet brakes.
Our minis use a simple gear pump and high pressures, plus experience medium-to high duty use. Those drive motors work really hard and generate a Lot of Heat. Back in 2018 when I was sharing experiences with 4-5 other Chinese mini-X "pioneers", a couple went naked on return filtration and had to replace their pumps after 100-200h.
I am just passing on what I have learned. Caveat emptor.