synthetic fluids

cuzncletus

cuzncletus

Senior Member
Is there an issue with running synthetic fluids in my 39 trans and diff? I've had very good luck with them in some older drivetrains though none as old as this. The difference in ease of shifting has been noticeable. A balky synchro ring that used to make me gingerly shift into second now shifts quickly and quietly. I was hoping for the same results in my old Ford driveline.
 
My dad has used STP in our gear boxes from 32 to 48, they never shifted smoother and better in their entire history. Just take the shifter top off and pour into the trans.
 
cuzncletus said:
Is there an issue with running synthetic fluids in my 39 trans and diff? I've had very good luck with them in some older drivetrains though none as old as this. The difference in ease of shifting has been noticeable. A balky synchro ring that used to make me gingerly shift into second now shifts quickly and quietly. I was hoping for the same results in my old Ford driveline.
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I would think they should be fine as long as they are "yellow" metal safe.
 
One thing to note is that in some cases the overall gear lube 'weight' might be a bit thin for an early transmission - as the clearances for the cluster and reverse main shafts can be enough to let the fluid seep out. There are no o-rings, gaskets or other sealing mechanisms to keep the lubrication fluids from running past the case holes where the shafts poke through. So it can depend on the overall case conditions, shaft conditions and whether or not the trans were rebuilt with shafts that are smaller in OD than stock (which almost all replacement shafts suffer from - a .002 smaller diameter).

You may find that the transmission tends to leak a lot more with a syn fluid - so just keep that in mind.
 
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