Drivetrain 1973 Mustang Tans Fluid And Rear End Oil?? Synthetic?

Discussion in 'Classic Mustang Specific Tech' started by Walter Sparks, Jun 22, 2013.


  1. Walter Sparks

    Walter Sparks Member

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    I just got a 1973 Mustang. I was told it has a 351 Cleveland and a C-4 Trans and a 9" rear. I am going to be changing the trans fluid and the rear end oil. Any suggestions on the best trans fluid and rear end oil. I use full synthetic in most of my vehicles, I noticed a huge difference when I changed to full synthetic in my 2000 F-250 rear end and manual trans.
  2. horse sence

    horse sence SN Certified Technician

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    never tried synthetic oil in a push rod motor but it made a big difference in my 2000 F150. a 73 with a Cleaveland should have a FMX or a C6 ,use
    type F trany fluid ,never gm fluid.
    Iamdiffrnt likes this.
  3. rbohm

    rbohm Founding Member

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    in older ford transmissions, using dexron ll isnt an issue. in fact using a mix of the gm and ford fluids back then softened the shifts slightly in the ford transmissions without hurting them. to the OP, if you want to use synthetic fluids on that old cleveland, go ahead. before you do though, make sure the engine is sealed up properly, meaning no oil leaks. after you change the fluids, if you find an oil leak, dont blame the fluids for it, chances are very good that it was already there just sealed up by sludge which the synthetic fluids do an excellent job of cleaning out of the system. as to which is best, follow the manufacturers recommendations for weight, as to brand pick your poison and go with it.
  4. Walter Sparks

    Walter Sparks Member

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    The engine looks to have fairly new gaskets. And it is pretty clean. so an oil leak would be easy to spot. So then it would not be good to use a modern synthetic tranny fluid?? Just stick with good old F fluid? I will use synthetic in the rear end, probably Amsoil 80 or 90W , I assume that will be ok.
  5. 65ShelbyClone

    65ShelbyClone Founding Member

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    Your wallet was substantially lighter?

    Conventional oils are so good anymore that I don't see a point in using expensive synthetic lubricants, especially not in an old architecture passenger car like a '73 Mustang. I relegate synthetics to special applications like extended drain intervals or extremely hard use.
  6. Walter Sparks

    Walter Sparks Member

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    yes it did cost about triple..so went from $50 to about $150 for the high end synthetic lube. but it is something that will be done only once every 5 years or so. So the money was not a huge issue with me, and with synthetic in the manual tranny and rear end, I guess I should get double the use out of it which helps justify the price difference.
  7. 65ShelbyClone

    65ShelbyClone Founding Member

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    The way I see it, if the percentage increase in cost doesn't provide at least an equal percentage increase in benefit like drain intervals or wear life, then it's actually not cost-effective to use synthetics. The oil manufacturers can provide suggestions for extending drain intervals, but really it should be done based on virgin and used oil analyses.
  8. rbohm

    rbohm Founding Member

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    go ahead and use the synthetic trans fluid. just use type f for best results.
  9. Walter Sparks

    Walter Sparks Member

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    Ok.. thanks guys

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