Nice information. I have to confess that I did run 90W gear oil in my B
transmission for a year or so back in the 80s. This was because it had
started to leak and I thought the heavier oil would not leak as much
until I could get it fixed. The transmission survived and gave me
another 20 years of service after that (unfortunately it is now in my
garage and needs to be rebuilt).
Still the properties cited are critical. I have had 90w gear oil
solidify in my Landcruiser transmission when temperatures got to -30 F.
I had to hold the clutch down for about five minutes or so after the car
started until the shaft could turn, even in neutral. That was also about
20 years ago - the Landcruiser survived and still has the original
clutch and runs well. And I now have had the fortune of having a house
with a three stall heated garage for the past 15 years.
-30 C is -22F whereas -24 C is -11F. Usually we don't see temperatures
much cooler than -20 F where I live in Montana but minus teens are
common during cold spells. I usually use 10-40w in the winter though and
it should have an even lower pour point.
I don't drive my MGs anymore when it is below zero F. Starting gets iffy
at that point and I have to scrape ice off the inside of the windshield
while I'm driving, even with the heat on.
David Councill
67 BGT
72 B
74 Toyota Landcruiser
01 Land Rover Discovery II
03 Volvo S60
-----Original Message-----
From: mgs-bounces+dcouncill=msubillings.edu@autox.team.net
[mailto:mgs-bounces+dcouncill=msubillings.edu@autox.team.net] On Behalf
Of mark.jones@exxonmobil.com
Sent: Wednesday, August 15, 2007 12:37 PM
To: mgs@autox.team.net
Subject: Re: [Mgs] Overdrive oil
The viscosity and other properties of 20W50 motor oil and 80W90 gear oil
are very similar. At 40 C (104 F), 80W90 gear oil (Esso Marvelube) has
a
viscosity of 149 cSt and 20W50 motor oil (Esso Extra) is 153 cSt. At
100 C
(212 F) gear oil is 16 cSt and motor oil is 17 cSt. Density (887 vs 880
kg/m3) and pour point (-30 vs -24 C) are also very similar.
The reason for different weights, 20W50 compared to 80W90, yet similar
viscosities at given temperatures, is because gear oil and motor oil
have
different grading systems.
So use either 20W50 motor oil or 80W90 gear oil in your MGBs
transmission,
either will work fine, at least that's what I believe.
Mark Jones
73 MGBGT
80 Spitfire
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