What is the capacity for Transmission Fluid on 1999 Mercury Mountaineer 5R55E 4x4 v6 engine got one book saying 10qts 5qts?

The discrepancy you're seeing (10 quarts vs. 5 quarts) for the transmission fluid capacity in a 1999 Mercury Mountaineer with a 5R55E transmission is likely due to whether it's a fill only capacity or a total capacity (including the torque converter).

* 5 Quarts: This is likely the amount of fluid needed to *top off* the transmission when it's already partially full. This is frequently what's stated for a drain-and-fill service.

* 10 Quarts: This is the approximate total capacity of the transmission, including what's inside the torque converter. A complete transmission fluid change (usually involving flushing or multiple drain-and-fills) would require roughly this amount.

To be certain, consult your owner's manual. The manual should specify the correct capacity based on the procedure being done (fill only or complete change). If you don't have the manual, you can usually find a downloadable PDF version online using the vehicle's year, make, and model.

Recommendation: When in doubt, err on the side of caution and use the lower number (5 quarts) for a drain-and-fill. Overfilling transmission fluid can be just as harmful as underfilling. You can always add more fluid if needed, but removing excess fluid is more complicated.