I have never seen a clogged valve. if the oil is that dirty it blocks the valve,then pump,struts and spheres all need replacing. The oil will last indefinitely as long as it is clean. ( I did 600,000kms in my W126 without ever having to change the suspension oil ) If it goes black ,thats a sign the rear seal in the pump has gone and engine oil is being sucked into the system. While that won't harm the struts,the heavier oil will cause the spheres to fail.
ned Kelly says his oil is foamy, Only one thing makes it go foamy and that is the gas from the spheres. As i see it,he is guessing what is wrong rather than simply checking the spheres First.
A worn valve will leak oil externally before any problems with ride height become apparent.
A worn valve Will not affect ride quality,it's a just a spindle with holes in it that open and close ports to allow oil to pass according to the position of the arm.
he says he can see oil coming back through the filter,the valve is obviously not blocked ( blockage is impossible considering we are talking of a pump which will reach pressures exceeding 2000 PSI,yes Two thousand) .
A worn pump will not lift a car,Simple.