Daniel1312@aol.com wrote:
>
> But does this mean that the gear oil gets much thinner at hotter temperatures
> than the engine oil of the same viscosity?
Likely no, but it might depend on a lot of things. I'll
leave that for another discussion.
A more or less proper statement set would be:
Engine oil running in the engine is much thinner
than gear oil running in a tranny, with both running
at the proper operating temperature for the
units.
However, when running at the same temperature,
the viscosity is roughly the same, so 20w50 engine oil
used in a transmission versus 80w90 gear oil running
in a transmission are roughly the same viscosity.
Note that the gear oil has a much more intense
additive pack, which does make them quite different.
The main additive that gear oils have (especially
hypoid oils) is something that promotes film strength
so it will still lubricate during the high pressure
squeegee-effect that happens on meshing gear
teeth.
So, especially in a differential, as the gear teeth
press against each other as they slide across the
tooth, the gear oil is much better at retaining a thin
film of protection.
Conversely, engine oils generally have additives that
make them better detergents, and are better at keeping
contaminants in suspension. In the engine, this is
important so that the oil can carry the gunk to
the filter and eventually out of the engine.
So, the best advice is, as always, use what the
manufacturer suggested. When you acquire a car, change
all the fluids anyways so you know when they've been
done.
However, if you find gear oil in the transmission as
seems to have been the case, I wouldn't worry about,
it likely did a fine job, especially in a non OD tranny.
A notable but mostly anecdotal exception to the above is
when it comes to REALLY OLD cars, where the manufacturers
suggestions were best then but not so helpful now.
A common example is that some very early car makers
eschewed the use of grease on some surfaces, preferring
oil. However, this had a lot to do with grease technology
of the time which tended to harden over time and stop
creeping into the vital surfaces. Modern greases tend
to work better and stay in place, so remember this next
time you buy an Austin Seven. ;>
There is also the "yellow metal corrosion" problems of some
gear oils, but so far this seems to be more of a theoretical
problem than an epidemic.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
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