Ask a busy workshop which gear oil to pour into a gearbox and you’ll often hear a confident “just use GL-5, it’s stronger.” That advice is wrong often enough to wreck expensive components. The GL-4 vs GL-5 gear oil question is one of the most misunderstood topics in the lubricant world, and getting it right protects your transmission, your differential and your maintenance budget.
Below we break down what these ratings actually mean, where each grade belongs, and how to avoid the single most common — and most damaging — mistake.
What GL-4 and GL-5 Actually Mean
“GL” stands for “Gear Lubricant,” and the numbers are API (American Petroleum Institute) service classifications. They do not describe viscosity — that is what grades like 80W90 or 85W140 tell you. Instead, the GL rating describes the level of extreme-pressure (EP) protection built into the oil.
That EP performance comes mainly from sulphur-phosphorus additives. GL-5 oils carry roughly double the EP additive concentration of GL-4 oils. More additive means more protection against the crushing, sliding loads found in hypoid gears — but, as we will see, more is not always better.
GL-4 Gear Oil: For Gearboxes and Synchromesh
GL-4 oil is formulated for the moderate-to-severe loads inside a manual transmission. Manual gearboxes contain synchronisers (“synchros”), and here is the catch: many synchros are made from brass, bronze and other “yellow metals.”
The sulphur-phosphorus additives in high-EP oils are mildly corrosive to yellow metals. GL-4’s lower additive level is gentle enough to leave brass synchros unharmed while still protecting the gear teeth. That is why almost every manual car, motorcycle gearbox and light commercial transmission specifies GL-4.
GL-5 Gear Oil: For Hypoid Differentials
GL-5 is built for the punishing conditions inside a hypoid differential — the type used in the rear axle of most trucks, SUVs and rear-wheel-drive cars. Hypoid gears slide against each other under enormous pressure, and only the higher EP additive load of GL-5 keeps a protective film between the metal surfaces.
If you run GL-4 in a heavy differential that calls for GL-5, that film can break down, leading to pitting, scoring and eventually gear failure.
The Costly Mistake: Using GL-5 Where GL-4 Belongs
Here is the mistake that sinks so many gearboxes: pouring GL-5 into a synchromesh manual transmission because “stronger is safer.” The extra sulphur-phosphorus additives attack the brass synchros. Over months, the synchros wear, shifting becomes notchy, gears start to crunch, and the gearbox needs an expensive rebuild.
The lesson is simple. Match the oil to what the manufacturer specifies. Bigger numbers are not better — they are just different.
What About GL-4/GL-5 Dual-Rated Oils?
Some modern gear oils are engineered to meet both GL-4 and GL-5 requirements using additive chemistry that protects gears without corroding yellow metals. These can be convenient for workshops that want to stock fewer SKUs, but you should still confirm the oil is genuinely yellow-metal safe and matches your vehicle’s requirement before use.
How to Choose the Right Gear Oil
Start with the owner’s manual or the OEM plate — it will specify a GL rating and a viscosity grade (for example, GL-4 80W90). Match both numbers: the GL rating sets the protection level, while the viscosity grade like 80W90 or 85W140 sets the flow characteristics for your climate and load. For Indian conditions — high ambient temperatures and stop-start city driving — using the correct viscosity is just as important as the GL rating.
If you are a distributor, fleet operator or workshop sourcing in bulk, buying from a manufacturer that clearly labels GL rating, viscosity and yellow-metal compatibility saves you costly mix-ups down the line.
Get the Right Grade from Petroblast
Petroblast Lubricants manufactures a full range of EP gear oils — including GL-4 and GL-5 grades in 80W90, 85W140 and more — from our facility in Mundka, New Delhi. You can explore our gear oil range or get in touch with our team for grade recommendations, bulk pricing and private-label options.
Choosing between GL-4 vs GL-5 gear oil comes down to one question: gearbox or differential? Answer that, match the viscosity, and you will get the full service life your equipment was designed for.