I am currently having the C4 trans in my '65 rebuilt at a local shop (with a rebuilt torque converter). The shop owner INSISTS that I use Dextron III in the tranny. I have read several posts on this forum, and it seems that some say "never use Dextron, only Type F" to "Dextron III / Mercon is OK". I don't want to mess-up a new rebuild with the wrong fluid, but shouldn't the shop owner know what he is doing? The rebuild carries a "3 year / 100,000 mile warranty", and I would think that the shop would NOT want to use / recommend a fluid that will destroy the trans. Any help / comments would be greatly appreciated.

 

If the shop who rebuilds provides 3 year warranty with that I'd do exactly as they say as otherwise you will have no warranty whatsoever

 

Type F has no friction modifiers and will result in a firmer shift when setup as OEM. Dex III has friction modifiers to soften the shift or at least smooth out a trans setup that would normally have a harsher shift due to design. If the rebuilder is using clutches needing a friction modifier, type F might be too harsh for a long life. Or the guy might not know what he's doing... I'd like to think he may be using clutch materials that require Dex III or Mercon. You might ask him if Mercon V is advised to reduce confusion later on as stepping up to a parts counter and ordering Dex III for a Ford can be confusing.

 

If you trust the shop enough to rebuild your transmission then use the fluid that they say to. There are several types of friction materials that can be used in the C4 and only the stock friction materials require the use of type F fluid.