Sulfur-phosphorus chemistry doesn't have a single answer. The right choice depends on whether you're chasing GL-4, GL-5, or MT-1 — and what your customer's transmission is made of.
EP additives sit at one of the most consequential cost/performance trade-offs in lubricant formulation. Too aggressive and you'll meet GL-5 specs but corrode the brass synchronizers in a manual transmission. Too mild and you'll pass GL-4 but score the hypoid teeth of a commercial vehicle differential. The right answer is application-specific and chemistry-specific.
Under shock load — say a truck pulling away from a stop with a fully loaded rear axle — the elastohydrodynamic oil film between gear teeth breaks down. For a microsecond, you have metal-on-metal contact at very high local pressure. Without an EP additive, that contact welds. Microscopic asperities tear away, and you get scuffing. Repeat that a million times and the gear is gone.
EP additives prevent this by reacting with the iron surface to form a sacrificial chemical film — iron sulfide and iron phosphate. The film shears more easily than steel-on-steel, so the asperities slide past each other instead of welding. You lose a few molecular layers of iron, but that's fine. The film replenishes.
Used in passenger car manual transmissions. The reason: passenger car manuals have brass synchronizer rings, and aggressive sulfur chemistry will corrode brass. GL-4 chemistry is calibrated to give enough EP performance for the load conditions (which are moderate — passenger cars don't pull trailers all day) without yellow-metal attack.
Typical Indian application: Maruti Suzuki gearbox oil, Hyundai/Kia manuals, Tata Indica/Indigo gearboxes. SAE 75W-90 or 80W-90 GL-4.
Used in hypoid axles, where the gear geometry creates aggressive sliding contact under high load. Most commercial vehicle differentials, tractor axles, and heavy-duty pickup truck rear axles use GL-5. The chemistry is more aggressive — higher sulfur content, more reactive phosphorus — and it will corrode brass synchros if you accidentally use it in a manual transmission.
Typical Indian application: Tata, Ashok Leyland, BharatBenz commercial vehicle differentials. SAE 80W-90 or 85W-140 GL-5.
The middle ground. Heavy-duty commercial vehicle manuals (Class 8 trucks in the US, equivalent in India) need EP performance beyond GL-4 but their synchronizers are designed for it. MT-1 chemistry is calibrated for this duty — and is increasingly common in modern HD manuals from the major OEMs.
Typical Indian application: Volvo, MAN, Scania commercial vehicle synchromesh transmissions. SAE 75W-90 MT-1 or 80W-140 GL-5/MT-1 combined-service oils.
OEMs increasingly want a single oil that lubricates both manual transmission and rear axle from a single fill — for service simplicity. This requires chemistry that passes GL-5 for the axle but is mild enough on yellow metals to satisfy synchromesh function. It's harder than it sounds; the additive package design needs careful balance of EP aggressiveness vs. copper passivation. Combined GL-4/GL-5 packages are available but cost about 15–25% more per kg than dedicated chemistries.
When in doubt, follow the OEM's lubricant spec. Or call your additive supplier.
We supply GL-4, GL-5, MT-1, combined GL-4/GL-5, and industrial CLP chemistries.