Regarding the viscosity question, I believe you have said what I said
already.
As far as 80W/90 in an overdrive, the word "virtually" is important here.
Also, BL was fond, as I pointed out, of making recommendations that they
later retracted. Further, this list has already repeatedly rebutted the
arguments in favor of using ANYTHING but engine oil in an overdrive. All of
the manuals I've ever seen, as well the Moss catalog (let's remember, Moss
has considerable access to technical and historical expertise both in the
States and Europe and England) every Brit car parts person and every gearbox
builder I've ever known say the same thing... Use anything but engine oil in
an overdrive at your own peril! Also, Volvo, who uses the same Laycock
overdrive in their 60-70's vintage cars, issue the same strong prohibition
against anything but engine oil in their boxes. So let's not go down that
tired thread again, you are misinformed and I hope not at the cost of a pricy
OD tranny.
As far as using ATF goes, the viscosity of atf is much lower than the nominal
30w oil recommended and again is intended for a different application. It
will not provide the same performance/wear dynamics as the intended lube.
The folks who built these boxes had access to the same atf we have today, if
they had thought it was a good idea they would have recommended it. Too
thin, too thick, or too slippery can all cause problems not discernable for
many miles, as well as too many or too few additives will. "Just cause it
works now doesn't mean it's right for later" is a good thing to keep in mind.
In a message dated 7/4/1999 4:24:47 PM Pacific Daylight Time,
gmccann@pcug.org.au writes:
<< 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
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