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?
> ===
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