A restoration pro discusses his experiences with transmission lubes and ZDDP

Question:

Id like to put in my two cents’ worth on the topic of “which ATF to use in my vintage automatic”.

Answer:

Id like to put in my two cents’ worth on the topic of “which ATF to use in my vintage automatic”. I own a restoration shop and part of my business is rebuilding vintage automatics. I strongly disagree that modern (non-synthetic) ATF will definitely result in leakage in a vintage automatic as asserted by a reader in the November issue. Any automatic which has not seen regular fluid changes is prone to leakage when the fluid is finally changed; likewise any automatic that has sat for an extended period of time is also prone to leakage. Having said that, if a tranny has been rebuilt properly, a modern ATF should not leak and in my experience will not leak unless there is some other underlying issue such as a worn torque converter hub or drive shaft yoke. As for compatibility, modern non-synthetic Dexron fluids are rated to be backward compatible with Type A fluid and I have not seen any issues with the trannies I have rebuilt.

While I’m at it, I don’t buy the sperm whale oil argument since beginning with the 1956 model year, GMC trucks began using motor oil which does not contain sperm whale oil) instead of Type A fluid in their Hydra-matics. It’s a practice which I have followed in my own ’55 GMC with Hydra-matic for over 12 years now without issue. It improves the shifting once the trans is warm, but cold shifting is a little worse.

I actually go longer between band adjustments with the motor oil. As far as I can tell, the clutch material in all of the pre-turbo 400 GM trannies is the same. I could be wrong but it sure looks and feels the same. I am NOT recommending motor oil in anything other than the truck versions of the Hydra-matic, as parts of the hydraulic systems are different than those in cars and the thicker fluid could adversely affect the operation of a car’s Hydra-matic. However, having said that I would not hesitate to try it in my own car if shifting is unsatisfactory with Dexron fluid, assuming I ever get the time to restore it. I suspect that the increased transmission failures after 1972 were due to the elevated operating temperatures of the engines because of emission controls. Those higher temps would be transferred to the transmission fluid through higher radiator temps.

While we are on the subject of motor oil, the current levels of ZDDP of 800 ppm, is exactly what was in motor oils from the mid ’50s all the way through the muscle car era and cams were not failing in great numbers back then (non-race applications, of course). ZDDP was increased in the ’70s not because of wear but because it is a cheap antioxidant and sludge was becoming a problem with the new emission controls and the resulting higher operating temperatures.

Also, too much ZDDP will cause your cam to fail rather quickly. That is probably why Lucas recommends their additive not be left in for more than 1000 miles. Shoddy engine builders and cam/ lifter manufacturers who aren’t getting the hardness of their products right have found a scapegoat and the old car public has bought it hook, line and sinker.

I run regular over-the-counter Carquest motor oil in my 455 Pontiac and recently had it opened up to replace a bad lifter and there was almost no wear on the cam or lifters after running it for a decade. Information on the history of ZDDP in motor oil is readily available online. For the record, I do use an additive during cam run-in.