The wear is from particles in the oil. The way it was explained to me is that the filter, filters everything larger than 10 microns. The smaller particles are free to flow in the system, and get in the smaller places, rotors, slide plates, etc. My filter is on the suction side of the tram pump, and therefore, the other pumps get the fluid directly from the tank As you bounce around on the PT, some of the finer particles are stirred up and contributes to more wear. I believe each pump should have it's own filtering. I will go ahead and change the oil and filter when I install the new pump. I am still not sold on the synthetics, although they seem to have some promise. The cost is double or tripled in some cases, but do they give double or triple the service?
I still have a Kohler engine on a Dixie Chopper that has 10 micron hyd filters for the hydraulics, and also a 1 micron by pass filter. It is hooks up in parallel. The engine was looked at after about 5 years, and they said it was as clean as a new engine. So I am thinking a filter like that would be a good idea.