The specification from MG for the gearbox in a MGB GT V8 with overdrive is
80W/90 so there goes the argument against using it. The GT V 8 gearbox is
virtually identical to the four cylinder MGB.
I have a letter from Castrol in which the Technical Manager states that
80W/90 is, in viscosity terms, thinner than 20W/50. Gear oils use a
different index. There is often the conclusion that 80W/90 must be
"thicker" than 20W/50 because the numbers are higher.
The main difference with gear oils that there have lower levels of
detergents, they work better under extreme heat and they have lower
frothing characteristics.
Having said all that it is not uncommon here in Australia to use automatic
transmission oil in the MGB gearbox.
Graham
97 MGF
At 02:06 pm 4/7/99 EDT, Johnmowog@aol.com wrote:
> Moss and most other manuals and British sources in saying that regular
>engine weight motor oil or something like Red-Line synthetic in an engine
oil
>weight is the way to go. The specific argument for 20w50 is that it will
>provide a more constant viscosity than a straight 30W oil, thus protecting
>your expensive gearbox innards under heat and stress as well as on first
>start-up.
>Regarding 80w/90 etc, you *can* probably get away with using almost anything
>you like, the B box is pretty sturdy, especially the 4 syncro., but the box
>is spec'd for engine oil. (Note: Using 80w90 in an overdrive will make your
>parts supplier and gearbox rebuilder very rich very quickly!)
> In reality, 80W/90 is actually close to the same viscosity as 30 weight
>motor oil, but the viscosity for hypoid oils like this is measured at a
lower
>temp range than motor oil so it "appear" to be thicker. Motor oil is rated
>"thinner", but that rating is done at a higher op temp. temp. In truth
>they're darn close to the same thing, but with different design temp ratings
>and additive packages. Multi-grade motor oils arguably have a greater
>effective temp. range than hypoid oils do, and greater "shear strength" as
>well. The box was designed to handle the additive packages typical in motor
>oil, not in hypoid oil, and this could be a concern. Depending on the hypoid
>oil, many have sulpher based additives not found in engine oil.
Sulpher-based
>stuff is corrosive to brass syncro rings, which we all have unless they've
>been uprated at some point. Whether this would cause a problem likely
changes
>depending on how much and what type of use your car gets.
>
>Whether or not you'll have a problem with the 80w/90 is not definate, and
>there are proponents of both approaches on the list. The different temp
curve
>may give seemingly smoother results in one climate while making it worse in
>others. The sent out a service note around 1976 or so suggesting changing to
>80w90, then recinded it months later after numerous dealer complaints of
>problems... for all we know climate differences may have been a
>factor...("Your Mileage May Vary")
> I've never seen the point in changing to a different oil. For
>whatever personal experience is worth, I have over 300K miles of MGB and
>Midget driving, all with 30w and 20w50, and my experience has been that I've
>never had a lubrication related failure.. or anything but top performance by
>the boxes. My preferance is to stay with what I know works...so I use engine
>oil, like God, MG and the Bentley manual recommend. :-)
>
>
>------------------original post----------------------------------------------
>> there have been several postings regarding overdrive transmission oil
>> but...what about standard, three or four synchro?
>>
>> the manual calls for the same weight oil as used in the engine, such as
>> 10W40. Others say it must be 30W. Others suggest 20W50, if there is a
>> problem. I have even seen suggestions involving diffential oil
>> (80/90).
>>
>> should things change now that our shift boxes are a couple of dozen
>> years old?
>> ===
>
>
Graham McCann Rivett, ACT. Australia
gmccann@pcug.org.au
Phone/Fax: +61 2 62889055
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