This product has been discontinued by Aeroshell but we are getting one more batch in soon. The Aeroshell Fluid 41 is the same material, MIL Spec 5606, but it meets the modern mil spec standard. It is the same fluid, just cleaner.
For more information, see the Aeroshell flyer about the change: Moving to Aeroshell Fluid 41 Flyer
Aeroshell Fluid 4 is a mineral oil based hydraulic fluid containing an oxidation inhibitor and a non-zinc anti-wear additive. It is dyed red for identification and leak detection purposes. It may be used in aviation and industrial hydraulic systems where very low temperature flow properties are desired and synthetic rubber seals are in use. It is not recommended for systems using natural rubber seals. APPLICATIONS Aeroshell Fluid 4 may be used in aircraft applications where a MIL-H-5606A (obsolete) specified hydraulic fluid is required. Aeroshell Fluid 4 may also be used in industrial and mobil hydraulic systems when extremely low startup temperatures are expected. Aeroshell Fluid 4 can also be used in hydraulic systems calling for non-zinc type hydraulic fluids in low temperature applications. NOTE: Use Aeroshell Fluid 41 in aviation hydraulic systems specifying MIL-H-5606G approved "super clean" hydraulic fluids PERFORMANCE BENEFITS Superior low temperature flowability Good anti-wear protection utilizing a non-zinc anti-wear additive Dyed red for easy identification APPROVALS Meets the requirements of MIL-H-5606A (obsolete) specifications for mineral aviation hydraulic fluids