Compatibility of GL-5 and GL-6 Gear Lubricants with Bronze

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    I?ve been told by the mechanic at a local antique auto museum that they use synthetic gear lubricants in the cars at the museum and are very satisfied with the results (it apparently reduces gear noise). I?ve also read that the sulphur in GL-5 and GL-6 (synthetic) lubricants will attack bronze. So I asked AMSOIL Tech Service whether they had any gear lubricants that did not contain sulphur. Here is their reply:

    The use of GL-5 r GL-6 lubes will not cause problems in antique vehicles (or any vehicle containing bronze parts), unless the temperature exceeds 275 Deg. F.
    Thank you for this opportunity to respond to your question(s). As always, please feel free to contact us again if we can be of further assistance.
    Byron Selbrede
    Technical Services
    The accuracy of this e-mail response is dependent upon the information provided. AMSOIL INC. is not responsible for wrong recommendations that were based on inaccurate or incomplete information.

    Any chemists out there who can explain why 275 is a magic number?



    I can’t explain the science but can only give you a personal anecdote.

    In preparation for a cross-country trip in my 1936 1601 in 1997, I wanted to assure the most efficient and cool-running lube for my differential. I’d heard rumblings about the effect of sulphur on bronze, so I called the Sta-Lube labs in Los Angeles and spoke with a Ph.D. chemist, whose name I no longer have. He said he saw no possibility of any adverse effects on bronze. So I installed Sta-Lube synthetic (GL-6). Six hundred miles into the trip, I crawled under the rear of the car to see how a leaky shock was faring (it was still leaking), but found the diff TERRIBLY hot. I then exited at the next town, and drained the diff and refilled it with GL-4. The draining GL-6 had the dreaded golden flecks in it, indicating that the bronze was disintegrating. I later re-drained and got only a very small amount of gold flecking, probably due to the residual GL-6 left in the unit after the first drain. The diff was much cooler with the GL-4. Ten years and ten thousand miles later, the diff is OK–but I am personally confident that it would have destroyed itself with the GL-6.

    I can only offer the Latin maxim "Quod dubitas non feceris," best translated as "When in doubt, DON’T."

    We all love to find modern products that will enhance the performance and longevity of our cars, but I am sworn off GL-5 and GL-6 gear lubes for life as a result of this experience.

    Best regards,




    I’m convinced. Thanks!!


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