Synthetic gear oil

GeoffGeoff Posts: 4,055Senior Contributor
Has anyone had experience with Synthetic oil?  Is it superior to mineral?   Penrite putout a synthetic 75W-90 which they state is  usable with copper-based alloys (bronze).  I'm wondering if this will provide better protection  and life for my Jet which has a B-W transmission and overdrive.   Any thoughts?


  • cchancelcchancel Posts: 157Member

    I can relay my experience.  Ken Ufheil recommended Royal Purple Max Gear 75W-140 ( on this forum a while back. I use it in the differential and overdrive transmission in my 1951 Super 6.  It meets the GL-4 and GL-5 specifications and is safe for copper alloys.

    The original differential was drained of its sludge and a unknown 75-90W gear oil was put in by a service shop.  After reading about potential corrosion problems with incompatible gear oil and a year of time, I drained as much as I could out and filled it with the Max Gear synthetic oil.  Less than a year after that I swapped the 4.55 assembly with a rebuilt 4.11 assembly (complete with new pinon seal and gasket).  The inside of the axle housing was surprisingly clean before the swap.  It was then refilled with the Max Gear.  To this day (2 years later) the differential has no leaks.

    The transmission seals have generally not been messed with since I bought the car.  It functions well, but it appears to be seeping more oil than it used to once the oil was changed over to Max Gear 75W-140.

    I then used Max Gear 75W-90 to replace the oil in my steering gear box. The steering box started seeping within 2 years.  At that point I drained it and switched to the flowable grease ( ) that Studebaker International sells.  No more leaks since.

    Generally speaking, my experience is like those that put synthetic oil in a car engine.  A new rebuild will stay clean and enjoy the befits of synthetic oil.  An old engine will lose the crud that helps keep the sealing surfaces intact and will leak.


  • GeoffGeoff Posts: 4,055Senior Contributor
    Thank you Chris.  I had heard that synthetic oil was bad for finding leaks.   I will give it a crack and see what happens.
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