In reply to David’s note above…..
Miller it seems produces two kinds of oil for elderly gearboxes. There is their “Classic Gear Oil” (which is a GL4 type) that David mentions and which was recommended by Miller. Then there is their “Green Gear Oil” (aka “Vintage Green Gear Oil” which is a GL1 type).
Below examples from the Millers web site of cars that might use their Vintage oils. All rather confusing. As a guide, the advice seems to be, use a GL1 type of gear oil.
MdeL
