Maybe I can shed some light to this issue. I taught industrial hydraulics for many years although I don't claim to be an expert.
The reason most hydraulic motors leak oil around the shaft seal is that they usually have too much internal leakage. (bypassing oil around the internal parts, much akin to blowby in piston engine rings created by internal wear usually caused by dirty oil.) No filtration on this oil or not changing the oil filter often enough when it is dirty.
Another reason for leakage is the shaft seal has been cooked by heat and is no longer pliable. Most of the causes for the shaft seal to be cooked is that the hydraullic motor has internal leakage and this bypassed oil is given up as heat which gets the hydraulic motor and case hot enough to cook the seal causing it to be brittle, wearing the seal lip down and or cracking the seal material. All hydraulic devices have a certain amount of internal leakage that is acceptable but excess is due mostly to wear caused by lack of filtration or dirty oil. A lot of hydraulic motors have three hydraulic lines to the motor, one is the supply from the pump to turn the motor, the second, usually as large as the supply is the return oil to the system and then the third is usually smaller to drain the leakage oil from the case back to the system.
Hydraulic motors are sized by cubic inches of oil used per revolution at a rated pressure.
They are categorized by their internal construction,I.E. gear motor, piston motor, and so on.
They can be interchanged by brand as long as the they are sized to the job being done and can be mounted in place. (Shaft size, Mounting brackets, hose location, cu. in./rev. max speed, etc)
