tktrain said:
Humm, Tucker told me to avoid GL-4 since it will attack yellow metal and GL-5 won't. Interesting to see the differences of opinion. Tucker still road races an MG, but I was specifically talking to him about Spits. He was quite adamant about no GL-4.
Are you sure it was "no GL4" and not "no GL5" ?? The latter is the "conventional" wisdom with regards to yellow metal corrosion. And GL4 is the factory-recommended oil.
The GL5 issue is actually not what "GL" spec the oil meets; but rather the use of a particular "EP" additive. That additive works somewhat similar to ZDDP, forming a microscopic coating that comes into play when the regular oil film breaks down under extreme pressure (and heat). Unfortunately, once the additive breaks down, the sulfur combines with moisture from the air to produce sulfuric acid, which can attack copper alloys (aka "yellow metal"). The acid also gives old gear oil that distinctive smell.
Both GL4 and GL5 oils are allowed to contain this additive; the GL specs are only performance specifications and do not require any particular formulation. However, some conventional GL5 oils use a much higher level of this additive (to meet the more stringent requirements of GL5), hence the warning not to use GL5.
But most (if not all) synthetic gear oils do not use the "active sulfur" additive, precisely because it does break down over time. Instead they use more expensive additives with longer lifetimes and no sulfur.
Unfortunately, most oil makers do not publish their additive formulas, so it can be difficult to tell if a particular oil contains sulfur or not. I believe that one good indicator is whether the oil is rated "MT-1". MT-1 itself is not applicable to our gearboxes & differentials, it is a standard intended for heavy duty long-haul trucks running non-synchromesh transmissions. But it does require that the oil exhibit long-term stability at high temperatures, so oils that use the "active sulfur" additive cannot meet MT-1.
I don't know about the TR7-8 differentials, but the TR2-TR6 diffs definitely use brass thrust washers for the spider gears. I have seen diffs where those thrust washers were fairly severely eroded (and black), so I believe the problem is real. But I have been using Valvoline full synthetic gear oil (which is rated both GL5 and MT1) in a variety of TR differentials for many years, and I have never seen any indication that it attacks yellow metal. My TR3A was already well worn when it came to me in 1984, and no worse when the car got wrecked in 2005 with another 200,000 miles gone by.
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