The various different types of gear oil confuse me. What does GL1, GL3, GL4, and GL5 etc. mean?

There are 4 levels of performance in classic gear oils and these really follow the development of gear design.

GL1 is the lowest level of performance and is only used in vintage transmissions. It is very similar to engine oil, having a very low additive content, but is available in much higher (thicker) viscosities to suit gearboxes with unsophisticated oil seals.  There are 2 products available in Millers Green Gear Oil range of GL1 oils as follows:  Millers Classic Green Gear Oil 90 GL1 and Millers Classic Green Gear Oil 140 GL1.

GL3 is the next level of performance and has very limited applications. Mainly specified for French and Italian (Fiat & Alfa) cars which had very soft synchromesh. It has very low EP (extreme pressure) performance and does not contain the concentration additives used in later engines.  We would recommend our product Millitran 4, SAE 50 for these applications.

GL4 is the first serious level of EP performance and is specified for many gearboxes and final drives from mid 1930’s onwards.  Millers produces the following GL4 oils:  Millers Classic Gear Oil EP 80w90 GL4 and Millers Classic Gear Oil EP 140 GL4.

GL5 specification is for highly loaded hypoid differentials as fitted to most cars from 1960 onwards.  Millers offer the following GL5 oils:  Millers Classic Differential Oil EP 90 GL5, Millers Classic Differential Oil 85w140 GL5 and Millers Limited Slip EP 90 GL5 for limited slip differentials.

To view and purchase products from our full range of Classic Gear and Transmission oils please click here.