Steve Gagnon wrote:

My understanding of the "difference in Ford vs. GM ATF is mostly historical.
Originally they had different additive packages, probably because they were
developed at different labs, and they had warrantee issues to think of.
Therefore, their recommendations were based on the lab findings that they
paid for and trusted.  They may be pretty close to being the same, but,
their legal departments are probably involved due to the warrantee thing.
I.e. Firestone tires

Hydraulic oil is NOT the same as mineral oil.  I don't think that mineral
oil has an additive package and may be cleaner burning.

The answer your looking for is not a one sentence thing.  In a nutshell, the
hydraulic oil was developed to survive a specific situation.  As hydraulic
controls work, they have relief valves, servo valves servo motors, and other
assorted valving (their getting more complicated all the time) in their
systems. Conservation of energy is a real physical constant out there.  When
a relief valve does it's thing, the "work" represented in the pressure, is
converted to heat.  When a hydraulic pump does it's thing, it creates heat
from the "heat of compression" thing, another physical constant.  When you
heat a machine up, all of the components expand a little with the heat.
When it is shut down, the darn thing sucks air into it as the components
cool and contract, which brings moisture from the atmosphere into it.  The
moisture condenses and becomes liquid and is in solution with the oil (it's
hydroscopic, it wants to absorb water).  Bottom line is that the additive
packages developed for these oils have become very specialized for their
applications, but they still basically do 2 things, protect the base oil
from heat and water in any form.

While were talking about additive packages, it's interesting to note that
there are only 2 or 3 major additive manufacturers.  One is Lubrizol.  One
is BASF.  And, I can't remember the other.  These companies may have changed
names a few times as corporate America goes.  The point I want to make is
that the additive packages can't be added to the oil like a can of STP.
They go in under "controlled environments" at the refinery.

I'm not familiar with TDH oil used on agricultural machines.  But, I'll go
back to one of my original statements, ATF's are basically hydraulic oil
with a red "leak detection" dye.  Therefore, I'll guess that if TDH was on
the scene before automatic transmissions, it is most probably a hydraulic
oil.  I'll speculate that the agricultural equipment of old does not have a
lot of sophisticated valving, but, I'll bet that the newer stuff does.  If
you try to maintain the new like the old, you may not like the results when
you are confronted with a $2,000 replacement for a servo motor or control
valve because .50 cents was saved on oil.

In summary, we really don't want 1 oil to do all applications.  That's where
this thing started from.  More and more specialized oils are being developed
every day. (think of what NASA and the military do today) I was really
hesitant to jump into the fray here because it really is a science unto
itself.  Bottom line is that you get what you pay for.  Not very witty, but,
true.  The oils that we depend on to keep us in the air are the "lifeblood"
of our machines.

The best advice I can give anyone is to change the oil - often.  It's
basically cheap insurance.

 
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