How to choose a gear oil step by step — a guide for service teams, maintenance and workshops

Unsure which gear oil to pour into a gearbox, reducer or axle? The right choice determines the life of gears, bearings and synchronizers, as well as noise level and energy losses. This guide walks you through the key decisions: viscosity, base oil type, additive package and OEM approvals. At the end you’ll find a short checklist and a link to the category page so you can pick a product right away. ✅

1. Define the application: industrial vs. automotive — not the same

For industrial gear units (worm, helical, bevel, planetary), viscosity is typically specified by the ISO VG class (e.g., VG 68, 150, 220, 320, 460) and additive requirements follow load and temperature. In automotive use, the SAE scale (e.g., 75W-90, 80W-90, 75W-140) and API GL classes (e.g., GL-4, GL-5) dominate, describing the level of EP anti-scuff additives. Mixing systems and specs can speed up wear or cause noise — this distinction is crucial.

2. Choose viscosity: operating temperature and speed are the main criteria

Viscosity sets the thickness of the lubricant film. High speed/low load calls for lower viscosities; heavy loads and higher temperatures need higher ones. Vehicles use multigrade ratings (e.g., 75W-90) to aid cold starts while keeping film strength under load. In industry, ISO VG 150–460 are common; the slower and more heavily loaded the gearbox, the more often VG 320 or VG 460 is selected. Remember: oil that’s “too thick” raises power losses and temperature; oil that’s “too thin” may not carry the load.

3. Pick the additive package: from EP to yellow-metal protection

Additives drive protection and stability: EP/anti-wear for scuffing, corrosion inhibitors, anti-foam and oxidation control. API GL indicates how much “EP” a gear oil contains: GL-4 is typically used in synchronized manual transmissions; GL-5 for hypoid axles under extreme loads. In many synchronized gearboxes, using a strong GL-5 can impair the work of brass synchronizers — unless the OEM states otherwise, GL-4 is safer there. In industrial gears look for “EP” and compliance with gearbox OEM norms.

4. Base oil: mineral, semi-synthetic or synthetic?

Synthetics (e.g., PAO) deliver better thermo-oxidative stability, longer drains and lower cold-start drag. Mineral oils are cost-effective for moderate loads and temperatures. Semi-synthetics blend both benefits and are often a cost-performance sweet spot. For continuous duty, higher temperatures or desired energy savings, a synthetic grade is worth considering.

5. Hydraulic-gear oil: when a 2-in-1 product makes sense

In agricultural/construction machines a single fluid may serve the gearbox and the hydraulics. Such products are marked UTTO or STOU. Choose a hydraulic-gear oil only when the design uses a common sump and requires a specific OEM spec. It simplifies lube management, but always follow the machine’s documentation — not every “universal” oil works with wet brakes, multi-plate clutches or complex valves.

6. Oil for hydrostatic transmissions: specific HST demands

Hydrostats run at high pressure and need stable viscosity and excellent filtration. Oil for hydrostatic transmissions should meet pump/motor/valve OEM requirements, often similar to HLP/HVLP classes with a high viscosity index and shear stability. Check seal compatibility and required cleanliness classes — excess foaming or contamination will quickly shorten HST life.

7. “Oil for steering gear” — clarify if it’s a mechanical gear or power steering fluid

Colloquially, oil for steering gear is sometimes confused with power steering fluid. That’s usually a different medium (e.g., ATF or specific CHF), not a typical EP gear oil. If you lubricate a mechanical gear without power assistance — follow the gear manufacturer’s spec. If it’s power steering — verify whether ATF of a given spec or a special CHF is required to avoid noise and pump failures.

8. When to choose a thick gear oil — and when that’s a mistake

A thick gear oil (higher viscosity) helps in slow, heavily loaded gearboxes where film load-carrying is critical. In modern, tight-clearance, high-speed units too much viscosity increases power losses, temperature and can worsen shift quality. Always follow the selection table and real operating temperatures — the right film must protect gear teeth without turning the oil into “grease.”

9. Quick selection checklist — before you order

• What type of gearbox and conditions (temperature, load, speed)?
• Which viscosity does the OEM recommend (SAE or ISO VG) and which API/EP class?
• Are yellow metals present (synchronizers, bearings) that call for GL-4 instead of GL-5?
• What drain interval and energy goals — is a synthetic base justified?
• Does one fluid serve hydraulics and gears — do you need a hydraulic-gear oil?
• Which OEM approvals/specifications are required?

10. Service & operation — pro tips

• Respect drain intervals — oxidized oil loses EP performance and accelerates wear.
• Watch level and leaks — losses mean more contamination and less protection.
• Sensitive systems (HST, precision gears) need clean breathers and filtration — replace filters on schedule.
• Don’t mix GL classes or viscosities without OEM permission; “topping off” at random dilutes additive balance.

What’s next? Select and order at Melkib

Go to the category page and filter by viscosity, base type and API class. If you need advice, describe the application, operating temperature and OEM spec — we’ll match a product to your use-case, and if needed invite you to our Showroom for hands-on testing. 🤝

➡️ Gear oils — browse products

FAQ — quick answers

Can I replace GL-4 with GL-5? Usually not in synchronized gearboxes with yellow metals; stick to OEM guidance to avoid impairing synchronizers.

Is “75W-90” always better than “80W-90”? No — they differ in low-temperature behavior. Choose per climate and OEM requirements.

When to switch to synthetic? With higher temperatures, long drains, energy-saving goals, or when the gearbox OEM specifies it.

Is a regular gear oil enough for hydrostats? Typically a dedicated oil for hydrostatic transmissions with defined cleanliness and viscosity stability is required — check documentation.