Rear Axle Lubricant

Have a 1997 Ford TB 4.6. Maintenance schedule book states that rear axle lube quantities must be replaced every 3,000 miles under severe duty schedule for non-synthetic fluid or 100,000 miles for synthetic fluid.
Question: Who knows what factory installs? synthetic or non-synthetic fluid?
Thanks for any input....
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quantities must be replaced every 3,000 miles under severe duty schedule for non-synthetic fluid or 100,000 miles for synthetic fluid.

Generally, Ford installs an extremely high quality lube in rear axle assemblies, of such good quality that they recommend that it never be replaced under normal service unless the axle has been submerged in water.
This is not unique to Ford, a friend told me about John Deere having to put out a bulletin advising farmers not to change axle lubricants prematurely, since the old lubricant they were draining out was much better even after use, than the replacement stuff was brand new.
I would buy whatever Ford's highest quality branded product is, presumably it will be a synthetic.
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axle lube quantities must be replaced every 3,000 miles under severe duty schedule for non-synthetic fluid or 100,000 miles for synthetic fluid.

fluid?
Mark
Thanks for the info. Will leave well enough alone. With only 36,000 miles on this car, no apparent leaks, never submerged. If it is good enough for John Deere it's good enough for Ford...... Again, thanks.
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