Asside from Ford telling me they are backwards compatable,
Here is what Texaco says:
"Havoline Automatic Transmission Fluid MERCON V is fully approved for Ford MERCON V and MERCON applications as well as General Motors DEXRON-III.
It is manufactured from select base oils and additives that provide oxidation and thermal stability, friction control, load-carrying ability, corrosion and wear protection, and prevent the accumulation of deposits and the formation of sludge, varnish, and foam.
Compared to MERCON fluids, Havoline Automatic Transmission Fluid MERCON V has exceptional low temperature flow properties and enhanced protection against viscosity breakdown.
Under the most severe operating conditions, Havoline Automatic Transmission Fluid MERCON V:
maintains friction control for smooth shift action
retains low temperature fluidity and high temperature stability for long operating periods.
protects automatic transmission fluid coolers from corrosion.
practically eliminates transmission overhauls due to sludge, corrosion, wear of clutches and bands, gears and bearings, leakage past seals, and loss of frictional properties.
Applications
Havoline Automatic Transmission Fluid MERCON V is designed specifically for use in Ford Motor Company transmissions that require a MERCON V fluid. The enhanced properties of the MERCON V specification offer premium performance in MERCON or DEXRON-III applications."
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Here is what Chevron says in addition to something like the above:
"Chevron Automatic Transmission Fluid MERCON V meets or exceeds manufacturer's performance requirements
Ford MERCON, MERCON V
General Motors DEXRON-III
Chevron Automatic Transmission Fluid MERCON V has the following qualifications:
Ford MERCON M000601
Ford MERCON V M5000601
General Motors DEXRON-III G-34555 "