I thought Mercon V replaced Mercon, and Mercon V can be used wherever Mercon is called for. But it seems I was wrong. Ford manuals state that Mercon and Mercon V are not compatible.
This is what I found in 2006 Focus manual:
Some transmission fluids may be labeled as dual usage, such as MERCON and MERCON V. These dual usage fluids are not to be used in an automatic transmission that requires use of the MERCON type fluid. However, these dual usage fluids may be used in transmissions that require the MERCON V type fluid. MERCON and MERCON V type fluids are not interchangeable. DO NOT mix MERCON and MERCON V.
Use of a transmission fluid that indicates dual usage (MERCON and MERCON V) in an automatic transmission application requiring MERCON may cause transmission damage. Use of any fluid other than the recommended fluid may cause transmission damage.
And I found this in my 2003 Crown Victoria manual:
Ensure the correct automatic transmission fluid is used. MERCON and MERCON V are not interchangeable. DO NOT MIX MERCON and MERCON V. Refer to the scheduled maintenance guide to determine the correct service interval.
These are found in the transmission section of the manual.
Furthermore, both manuals state that i should use Mercon, not Mercon V, as power steering fluid.
Both cars specify Mercon V for the transmissions. So, according to both manuals, I should only use Mercon V. That makes sense, because Mercon V is newer, so I shouldn’t use the older stuff in it. But the Focus manual states that I shouldn’t be using Mercon V in older cars that require Mercon. That must mean there is some difference in friction characteristics or something else that makes a difference in hydraulic uses.
Then, it should follow that I should use Mercon in the power steering and NOT use Mercon V in it.
I know of Ford mechanics who say that Mercon V is backwards compatible, so I should be able to use Mercon V wherever Mercon is called for. Did Ford write this in their manual for more than a decade but never told their mechanics?