There was a point in the mid-70s, when British -Phleghm-sucking-Leyland was
in charge of things, when they sent out service bulletins to the U.S,
suggesting we run our gearboxes with 90w oil. This was doubtless due to some
poor saps in Texas cooking a gearbox after driving across the whole state in
overdrive 3rd on a 110degree day. Needless to say, after a flood of delighted
dealer service departments and very angry gearbox owners collided, B(ps)L
recinded said service bulletin and returned to their advice as originally
given e.g: a high quality non-synthetic motor grade oil like 20w50 Castrol.
Some manuals that were researched during B(ps)L's regression into the instant
gearbox rebuild business gathered this wrong info and have clung to it to
this day. The rest of us drive our happily lubricated boxes full of motor
oil.
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