Originally Posted by JimPghPA
My understanding is that one of the big things that degrades fluids is exposure to air, and the very small amount of moisture that condenses out of the air and into the fluid. Over time that can add up to a lot of oxidation and moisture. When you have a dip-stick every time you go up or down a mountain, or the barometric pressure changes a small amount of air moves in or out of the air space above the fluid in your trany. When it moves in it carries new oxygen and water molecules (moisture). Some of that interacts. A sealed system does not have the fluid degrade anywhere near as fast.
I have seen some engineering specifications for some fluids and there useful life that specified different life expectancy for use in sealed and unsealed systems.
The first "only right" answer. And in this competitive days....no car company builds planned obsolescence into their vejicles if they want to stay in business.
This begs the question: How can ATF last the lifetime of the vehicle? The answer has everything to do with efficiency. As complicated as the automatic transmission has become, it's also become far more efficient. Just as engine design and lubricant quality have enabled the use of thinner oils that last longer and deliver better fuel economy and performance, automatic transmission design has made similar advances. As automatic transmissions are more efficient and built to tighter tolerances, ATF doesn't heat up as much, reducing wear and oxidation. Older automatic transmissions needed service every 30,000 or 60,000 miles, but the ATF in newer automatic transmissions can last upward of 150,000 miles without issue.