"It is essential that an approved lubricant be used when refilling,
preferrably a straight mineral oil with a viscosity between SAE 30 and
SAE 50 with no E.P. additives.
ON NO ACCOUNT SHOULD ANY ANTI-FRICTION ADDITIVES BE PUT INTO THE OIL."
Right. And we've recently determined, in this thread, that the gear oil and
engine oil viscosity scales are different, and that the gear oil viscosities
that correspond to those mentioned above are in the 75-90 range.
Note the phrase "straight mineral oil". That means, in particular, no
multi-viscosity oils, and no additives. Engine oil these days is almost
exclusively multi-weight (yes, you can find it straight) and has a witch's brew
of additives to help foaming and to minimized the deleterious effects of blowby
on white metal bearings.
In short, it's hard to *find* the "old formulation" oils, at least if you look
at the engine oil shelves. (Want a challenge? Find a modern oil that
corresponds to Shell Donax 250 weight gear lube from the late 60's!)
I recommend that you use what has worked for others, and not try to second
guess what a 30-40 year old technical publication means in today's oil market.
I've been very happy with modern GL-4 rated gear oils in my Laycock overdrives
in Triumphs. I don't plan to change soon (though I'm often tempted by a
synthetic that won't thin out so for the D type in my GT6+; I'll probably solve
that problem by adding some cooling).
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