From: colfelt@ucsu.Colorado.EDU (Andrew BW Colfelt)
Subject: Re: Engine oil in gearbox (was synthetics)
Organization: University of Colorado, Boulder
Date: Mon, 12 Oct 1992 05:33:00 GMT

shaman@cix.compulink.co.uk (Leo Smith) writes:

>colfelt@ucsu.Colorado.EDU (Andrew BW Colfelt) writes..

>>Using any kind of motor oil in the gearbox is preferrable to heavy
>>gear oils becuase of the detergent and anti-sudsing additives. It
>>would be wise to change this gearbox oil more often than heavy gear
>>oil because of shearing. It would also be wise to use motorcycle oil
>>in this application because of the anti-shear additives.

>I think that despite your qualifications, I disagree: Gear oils are
>heavily optimised for gear usage. Motor oils are not.

This was my feeling as well, thinking that the 
dealership mechanic was trying to make my gearbox age
rapidly so that he could fix it.  

Since there is a lot of metal shaving-producing friction in
the gearbox, there is a lot of debris that builds up.
While I cannot directly refute your statement that gear oils
are heavily optimized for gear use, I can state that it is
my belief that heavy gear oil does not contain detergent
additives that hold debris in suspension, nor does it contain
anti-sudsing, 'anti-scrubbing bubble' additives.  True, gear oil
may be highly anti-shear, but it is heavy and causes more drag,
smells bad, and, to my knowledge, somewhat old school.

To summarize:
		PRO			CON
		===			===
Motor Oil	Detergent		Requires short change interval
		Flow better when cold	Not Anti-shear
		Smells good
		Anti-sudsing

Gear Oil	Anti-shear		smells bad
		Long change interval	flows poorly cold
					no detergent
					no anti-suds

To address the CONS of both products, I would suggest that
the inclined motorhead use MOTORCYCLE engine oil in the
gearbox, as this stuff is meant to be used in in HARSHEST
of gearbox environments, i.e the 15,000rpm teeth-meshing, 
oil-shearing, shreddin' whinin' machine.

> Synthetic gear
>oil is probably the best if anyone makes it.

Whew!  I thought, after reading the subject line of this
post that someone had just blasted my credibility.

Thusly, I HEARBY TOTALLY AND UTTERLY *RETRACT* A STATEMENT
I MADE IN THE PREVIOUS POST!

DO NOT, under any circumstances, use SYNTHETIC oil in a
SYNCHROMESH GEARBOX.  Synthetic oils are so slippery that the 
synchros cannot generate enough friction to synchronize
gear and shaft speeds.  Of course, gearboxes that do not
use synchros, such as a motorcycle gearbox, do not have a 
problem with synthetic oils.

>It is not just a question of the oil lasting longer, it is a
>question that the motor oil might actually break down under high
>shear and wreck your transmission.

This addresses a major CON to the motor oil argument, and the
major PRO to gear oil.  However, if one changes motor-oil-in-
the-gearbox synchronously with the engine oil (3000miles),
the breakdown should not pose a problem.  Furthermore, the
additives in motor oil that suspend debris and fight sudsing
allow motor oil to protect the gearbox better than gear oil,
albeit for a fewer number of miles.  And, to repeat, motorcycle
oil is everything regular motor oil is, plus the anti-shear
additives, so it is really the best solution all around.

And to totally exhaust this subject: Those of you who are
interested in the benefits of synthetic oils should be 
aware that it is NOT wise to use synth. oil in a brand new
motor.  Petro oils are recommended for break-in periods,
as synths do not allow rings to seat properly (again, too slippery).


-- 
Andrew          
Colfelt@ucsu.Colorado.EDU
--