by jbcollier » Mon Jan 14, 2019 3:23 pm
GL4 gear oil does not affect brass/bronze and is used in Land Rover transfer cases. It's not easy to find but it is available. I use Pennzoil 75w-90 GL4. The most readily available gear-oil is rated GL5. It is used in limited slip diffs with clutch packs, though sometimes with an additive. I have used GL5 rated gear oil in many, many Vespa and Lambretta (chain-drive) gear-boxes with no problem. There isn't much bronze in the later Lambertta/Vespa gearboxes. No effect on the clutches.
Bevel drive Lambrettas are a different matter. They do have a number of bronze bushings in the gear-box, typically have long periods between overhauls, and are often neglected between overhauls. I would not use GL5 in a D/LD gearbox. Fortunately, you are not supposed to. It specifies 30 weight engine oil, changed frequently. You do use gear oil in the final drive (120 weight IIRC) and EP additives are very important there. You could use a GL4 gear oil in the D/LD gear box. It would work very well (much better for the bevel gears so you wouldn't have to change it so often) and is only slightly heavier than the specified engine oil. Gear oil viscosity is measured differently from engine oils: 80w90 gear has roughly the same viscosity as a 40 weight engine oil.