Difference in application between API GL-4 and GL-5 transmission oils

19 September 2013

Question: It is very common to assume that all GL-5 oils are an improvement over the GL-4 oils and can completely replace them. But is this the case?

MITASU OIL expert help: This statement is true only in part, because the API classification categorises gear oils depending on the content of anti-wear, anti-scoring and other additives in its formula and depending on its rating for EP (Extreme Pressure) protection. API classification does not provide the separate rating for synchromesh transmissions, which demand an additional requirement for compatibility of oil with non-ferrous metals. GL-5 oil may well replace the GL-4 product in a cross-axle differential (in the speed reduction unit or in the axle), and at the same time, prove to be unsuitable for the manual (including sequential) or synchromesh transmissions.

Question: But GL-5 oils designed for higher EP protection than GL-4, so why is oil with a higher ability to withstand pressure may be unsuitable for the manual gearbox?

MITASU OIL expert help: The reason is that GL-5 oil formula includes a powerful group of additives, commonly containing additives based on phosphorus and sulfur, causing the excessive wear of synchronisers, made from non-ferrous metals.

Question: Does it mean that only API GL-4 oils should be used in synchromesh transmissions?

MITASU OIL expert help: No, it does not. As a rule, GL-3 or GL-4 oils are used for synchromesh transmissions, however, some vehicle manufacturers, such as Subaru or Alfa Romeo, for example, use GL-5 oils.

Question: In that case, which criteria should be used for selection of transmission oil for a vehicle?

MITASU OIL expert help: Frequent use of innovative technical solutions in the production of transmission units and components by car manufacturers, does not allow for a universal method to properly match transmission oils. Primarily, the vehicle manufacturer's guidelines shall be used where available. Always pay attention to the application range specified on the label of the transmission oil. When it is not possible to identify the purpose of the lubricant, it is best to refer to specialists. MITASU OIL dealer centres have specialised catalogues to assist in choosing the right lubricant for your vehicle.