The question of what lubricant(s) to use in Bullet 4-speed "Albion" gearboxes came up very recently in a separate thread and has been much discussed in several threads of the past.
Perhaps the definitive examination of the matter is found in a superb video by Paul Henshaw (a.k.a. "The Bullet Whisperer") of
Performance Classics at
https://youtu.be/PaD3wfwu9Gs.
In short, the original Redditch gearboxes called for oil. Yet apparent lack of effective oil sealing in later Indian-made gearboxes led to a practice of filling them with a
very thin grease known as "00" and topping off periodically with gear oil, typically of about 90 weight. This was frankly a bit of a slapdash workaround or "bodge" to reduce oil loss from poor sealing. Later Indian-made 4-speed boxes with improved seals or earlier boxes later retrofitted with same (available from our Forum hosts Hitchcocks) generally reverted to the original British oil-only/no-grease regime.
Generally whatever weight oil is used should probably NOT be "GL-5" rated, as the phosphoric anti-impact additives used in some (but not all) gear oil formulations, which may note on the jug or bottle that they are "compatible with grades GL-2 through GL-5" are probably best avoided as they MAY be corrosive to any copper or brass bits found within. If not found among the automotive offerings, straight GL-4 gear oil can often be found among boating supplies, as it is often called for in outboard motors and outdrives. Mercury's
Quicksilver brand SAE 90 is a fine choice. That said, the estimable Mr. Henshaw says in his video that he simply uses 20W-50 engine oil. I chose the straight GL-4 90 weight simply because that's what's called for in my latter-day 5-speed's
Owner's Manual.
For most owners who have not recently refurbished their 4-speed gearboxes, which are working "well enough", I might recommend simply draining and refilling with straight GL-4 SAE 90, whether conventional or synthetic. If it leaks a bit more than you'd care for--quite possible with a 1990s or earlier box with slapdash seals--maybe squirt a bit of that very light "00" grease in there as a "chaser" like the Indians do. The stuff is readily available online.