"What is the best way to check the sludge formation tendency of a new hydraulic fluid?"
ASTM D2070 is a test method that's "designed primarily to evaluate the thermal stability of hydrocarbon-based hydraulic oils, although oxidation may occur during the test." This oxidation may result in sludge, which is measured and recorded in milligrams per 100 milliliters.
This description of D2070 could lead you to believe that the test will provide an adequate indication of the sludge formation tendency of a mineral-based fluid. However, you must also consider the scope of ASTM D4310, which states, "This test method covers and is used to evaluate the tendency of inhibited mineral oil based steam turbine lubricants and mineral oil based anti-wear hydraulic oils to corrode copper catalyst metal and to form sludge during oxidation in the presence of oxygen, water, and copper and iron metals at an elevated temperature. The test method is also used for testing circulating oils having a specific gravity less than that of water and containing rust and oxidation inhibitors." This test is specifically designed to quantify the sludge formation tendency of a mineral-based steam turbine oil or hydraulic fluid.
It should be noted that although D2070 will give an indication of the sludge content due to oxidation of the copper and steel rods, it is more of an acknowledgement of the sludge as a byproduct rather than a test specifically for sludge. On the other hand, D4310 is exclusively a test for the measurement of the sludge formation and corrosion tendency of a mineral-based turbine or anti-wear hydraulic fluid.
It would be easy to confuse these two tests, as they are seemingly quite similar. However, the D4310 standard is more of an involved test procedure than D2070, which uses copper and steel rods in combination with heat. D4310 utilizes copper and steel coils in combination with water and heat to oxidize the oil for the purpose of forming sludge.
Procedure "A" of ASTM D4310 requires the determination and report of the weight of the sludge and the total amount of copper in the oil, water and sludge phases. Procedure "B" of ASTM D4310 involves the sludge determination only.
Keep in mind that while the D2070 test method can lead to the formation of sludge, the D4310 test procedure will result in sludge formation. Although the two tests may seem similar, they are examining two different properties of a mineral-based fluid.