John :
This is always a troublesome topic, there doesn't seem to be a lot of
agreement or consistency about it. The issue is that some gear oils use a
sulfur-based additive that increases the "extreme pressure" properties of
the oil (basically the film strength). Under some circumstances, some of
the additive can break down, and the sulfur can oxidize and combine with
water from the air to form sulfuric acid (H2SO4), which can corrode brass
parts over time. Even most conventional GL4 gear oils have some of the
additive (it's what gives them that distinctive smell), but most
conventional GL5 gear oils have more.
Unfortunately, the GL4/GL5 standards do not (adequately) address the issue
of how much of this particular additive is in the oil, so a GL5 rating does
not necessarily mean the additive is present in harmful amounts, and a GL4
rating does not necessarily mean it's safe. From reading the standards, it
appears that any GL5 oil would most likely also pass the GL4 tests, I
believe the only reason that more oils don't bear the GL4/GL5 rating is
because the makers don't want to bother with two sets of tests. According
to Redline, they do not use this additive at all in their GL4 rated
synthetic "Manual Transmission Lube", and only small amounts, comparable to
a conventional GL4 oil, in their GL5 rated synthetic gear oils.
The other issue is those "certain circumstances". I haven't yet heard of
anyone that has actually had a significant amount of corrosion from using
GL5 oil in a TR tranny. The brass parts do turn somewhat dark, but that
doesn't seem to hurt anything. The story apparently originated in a
completely different car (Ford Model T ?) that has brass parts in it's
differential. It may also depend on your driving habits, it apparently
takes a good deal of heat and pressure to cause the additive to break down.
I rather suspect that most street-driven TRs never get into the required
conditions.
So, I'm not sure there's anything to worry about in a TR, but if there is,
it's only in the transmission. There are no brass parts in the
differential.
Even if you were having corrosion from the GL5, that would not cause visible
pieces in the oil. It would take a chemical analysis of the oil to detect
the problem without tearing the gearbox down for inspection.
Randall
>
> I have a later non-OD TR-6. Does anyone known if the tranny and/or diff
> have yellow metal parts that I need to be concerned about?
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