API GL-3 gear oil is no longer in production. Despite this, as of December 2011 some scooter manufacturers are still delivering their products with advisements in the operating instructions, or stickers on the machines, that mandate the use of GL-3 gear oil. Upgraded options are now marketed that are improvements on GL-3 in every way.
The American Petroleum Institute -- API -- uses GL ratings to classify gear oil. Prior to making a decision in 1995 to phase out obsolete products, the API classification system divided transmission oils into six categories: API GL-1 was recommended only for the lightest applications, such as worm gears in slow-moving farming machines. API GL-2 was recommended for moderate conditions, such as the slightly faster moving transmissions in tractors. API GL-3 was also for moderate conditions. API GL-4 worked well in various applications, from light to heavy duties in the gearboxes and axle units of trucks and buses. The old API GL-5 was recommended for severe conditions -- high speeds of sliding where significant shock loadings occurred -- in transmissions and limited slip differentials. API GL-6 was also for heavy-duty applications.
GL-4 is an upgrade to GL-3, and supersedes the latter’s applications; it works well in almost all modern gearboxes. GL-5 is not an upgrade to GL-4, and is manufactured for specific applications; it works better, for instance, in transmissions with separate differentials. It is unsuitable for use in gearboxes with synchromesh mechanisms. Since scooters do not have synchros, it is acceptable to use the new GL-5 in their transmissions. Because fully synthetic oil runs somewhat cooler than traditional types, a fully synthetic GL-5 is preferable to a traditional GL-4. In any case, GL-4 is also on the obsolete list, and becoming increasingly difficult to find.
It is worth noting that gear oils have a thinner viscosity than engine oils with a comparable weight rating; 75W90 gear oil has a similar viscosity to 10W40 engine oil, for instance.
For applications where GF-3 or GF-4 oils are mandated by the manufacturer, the new GF-5 is now recommended. With its introduction in October 2010, GF-5 superseded all the obsolete ratings. It has superior sludge control, enhanced emission control, is compatible with all seals used in engines manufactured in 2011 and before, and works well with engines running fuels containing ethanol.
Synthetic 75W90 gear oil, with an API rating of GL-5, is an entirely serviceable alternative to any outdated recommendations made in manufacturer literature, and is often quoted as the best choice in user service documents. As user demands and manufacturer technologies differ, always take a scooter using an alternate gear oil for a gentle test ride before putting it into regular service.
After riding a brand new scooter for approximately 1,000 miles, change the gear oil twice; this will remove any tiny metal particles that have been shed by the gearbox during the break-in period. There are scooterists who then never again change the gear oil for the life of the scooter, and others who advocate changing it along with every third engine oil change. Unless the manufacturer’s instructions or warranty conditions give specific instructions, user preference is the main determining factor.
