Originally Posted By: Mike_dup1
fyi
from the bottle of Dexron VI
Here is the mentioned
GM service bulletin.
The main difference between Dexron-VI and Dexron-III is the fresh-oil viscosity, 6.4 and 7.5 cSt, respectively. However, Dexron-VI is synthetic or synthetic-blend and therefore uses less viscosity-index improvers, which means less shear. As a result, they both have the same lower limit on the viscosity after oil shear (for used oil), which is 5.5 cSt.
The concern for manual transmissions and PS vane pumps would be the 15% lower fresh-oil viscosity, which may result in a little less wear protection. However, the difference is not that great and these systems work in boundary-lubrication regime (metal-to-metal contact) anyway, and in this regime the main wear protection is provided by the antiwear additives such as ZDDP and moly. It wouldn't be a great concern if you used Dexron-VI in a manual transmission or PS, especially a fully synthetic one such as Valvoline Dexron-VI. For peace of mind, you might want to use an appropriate manual-transmission fluid in a manual transmission. PS system is probably even less of a concern and if you really think you may need a tad bit higher viscosity, stick with Dexron-III then. In my opinion chances of increased wear due to this slightly lower viscosity is small -- after all, they wouldn't risk expensive automatic transmissions without being sure that the lower viscosity is providing the same protection.
To me, the warning on manual transmission and power steering sounds like they just didn't want to bother to test and recommend the Dexron-VI on these systems and chose the easy way out of not recommending it instead of spending a lot of money to test it so that they can recommend it for those systems as well.