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I don't have a lubrication guide for this range of Triumphs to hand, so I can't answer with absolute authority.
With many models of Triumph, one is inviting serious problems by using GL5 gear oil in the diff.
Most manufacturers formulations of GL5 (there are one or two exceptions, I believe Penrite are one exception, but check first) contain chemicals that attack and disolve phosphor bronze bearings and shims - and most of the smaller Triumphs use phosphor bronze components in the diff, and therefore require GL4 gear oil in the diff.  I don't know about the big Triumphs.
To be certain, you would need to check the lubrication guide - if it doesn't list GL5 as being suitable, do not use it - it is not better than GL4, it is different !

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It's not a case of EP90 or something else.

Oil comes in various thicknesses and specifications

EP is "Extreme Pressure" ie gearbox (and trunion) oil
80, 90, 140 etc is the viscosity, and if there are two numbers quoted, such as 80W90 then when cold it behaves as an 80 viscosity oil, and when hot as a 90.

There are two grades of gear oil generally available
API-GL4 which does not attack phosphor bronze
API-GL5 which normally destroys phosphor bronze

GL5 is not better than GL4, it is different, and has different properties.  The diffs on most (but not all) modern cars have been designed to work with GL5

According to the RimmerBros website, the correct oil for a T2000 diff is Unipart GGL190, which is a GL4 specification oil EP90 oil - they specify the same oil for the small Triumphs as well, so I guess that the T2000 diff also contains components that are destroyed by the additives in GL5 oil.

Checking the lubrication chart for a T2000 should confirm that a GL4 specification EP90 oil should be used.  If the chart says GL4, then DO NOT use GL5 unless the manufacturer guarantees that it is compatible with internal components of the diff (Penrite is the only GL5 oil that I think is compatible)  If GL4 spec EP90 cannot be obtained, the GL4 spec EP80W90 is fine.  Halfords sell GL4 EP80W90.

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