1603 Differential gear oil recommendations

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    Hello, I am doing an annual flush of all my fluids, and am having a difficult time finding what I previously used in the differential for my 1936 1603. I believe it was a Napa branded Gl-4 fluid, but I can no longer find it on their website. They have a Sta-Lube product that is labeled as GL-4, as well as a LiquiMoly product labeled as GL4+/GL5, but I am not sure if this is safe to use in the differential.

    Any recommendations for brands/types of fluid are appreciated.


    Chris Williams



    Sta-Lube GL-4 is available through NAPA, smaller stores may have to order it. We’ve used it in our 1601 for the past 9 years. Stu Blair



    The Sta-Lube Stu mentioned should work fine for you. Redline and Chevron (Caltex) also sell GL-4 Gear Oils. I have used Sta lube gear oil for many years in a variety of vehicles and never had a problem.

    The main difference between GL-4 and 5 is the amount of Extreme Pressure additive (sulfur/phosphorus), which can be bad for yellow metals. I think I would stay away from the GL-4+/5 oil although I don’t think it would hurt your differential, but why take the chance. Most of the problems with GL-5 have been when it is used in a transmission that has brass synchro rings. It either causes corrosion of the brass, or the different friction characteristics cause poor shifting. Hypoid differentials such as yours require an EP type (such as GL-4) gear oil- Dave


    Thank you for the information guys. I ordered a 5 gallon bucket of Napa GL-1 for the transmission & overdrive unit, and the Sta-Lube GL-4 for the differential. I hope to get them changed out this weekend.

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