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TES-668
Latest Allison long-life spec. Use in modern Allison 1000 rebuilds.
When to use: Replaces TES-295 in most applications. Better high-temp performance.
Wrong-fluid consequence: Generic Dexron in an Allison = immediate slip + warranty void. TES-rated only.
TES-295
Previous Allison spec. Still in service for older units.
When to use: Use only if specifically called for and TES-668 is not approved for the application.
Wrong-fluid consequence: Same as above - never substitute Dexron or other Mercon-class fluids.
Allison fluid is expensive ($25-40/qt) but mandatory. Skipping the right fluid costs you the entire rebuild within 20K miles. Source from Allison dealer or specialty diesel shop.