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
From: Bob Howard <mgbob@juno.com>
Subject: Re: [Mgs] Overdrive oil
Why would anyone use 90W gear lube....?
MG at one time issued a Tech Service Bulletin that said 90w could be
used. About a month later that TSB was rescinded, but that bad information
was picked up and used in some service manuals published by others. I have
an old Haynes that shows 90 as an approved lubricant. It came with an
OD-equipped MGB when I bought the car. I changed all fluids as soon as I
got the car, including changing to 90 from whatever nasty sludge was in the
OD at the time, and it worked fine until air temp dropped below about
60degrees F.
Sad to say, that erronious information is still out there in some old
books.
Bob
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