Walk into any auto-parts importer's gear-oil aisle and you will find the same two letters on the bottles: GL-4 and GL-5. They sit like neighbours on a shelf, and most buyers assume the higher number is simply 'better'. It is not. For manual transmissions — the boxes that take 75W-90 — choosing GL-5 where the maker calls for GL-4 can quietly eat the synchronisers. This is the single most common gear-oil mistake in the import trade, and the one that generates the angriest warranty calls. Here is how to put the right bottle in the customer's hand.
What GL-4 and GL-5 actually mean
Both belong to the API gear-oil classification, which grades oils by the load they can carry and the components they can protect. The difference you care about is the extreme-pressure (EP) additive package — the sulfur-phosphorus compounds that let the oil survive the enormous sliding loads inside a hypoid final drive. GL-5 carries roughly twice the EP additive level of GL-4. More EP additive means more load capacity; it does not mean the oil is 'stronger' in every way that matters to a gearbox.
| Property | GL-4 | GL-5 |
|---|---|---|
| Typical use | Manual transmissions, transaxles, light-duty axles | Hypoid differentials, heavy-duty axles, some transaxles |
| EP additive level | Lower | Roughly 2× GL-4 |
| Sulfur content | Lower | Higher |
| Load capacity | Moderate | High |
| Risk to brass / bronze | Low | Can attack yellow metals |
The copper-corrosion trap
Here is the part that catches importers out. Many manual gearboxes — especially older and Asian-built units, and a lot of the light-commercial and small-truck boxes popular in hot-climate markets — use brass or bronze synchroniser rings (yellow metals). GL-5's higher sulfur EP additives, excellent at protecting hypoid gears, are aggressive toward those yellow metals. Fill a GL-4-spec'd box with GL-5 and the synchronisers can corrode and wear, shifting gets notchy, and the box fails early. The oil did its job on the gears and destroyed the part the maker was protecting with the GL-4 call.
This is not a corner case. It is why a surprising number of vehicle makers specify GL-4 for the manual box and GL-5 only for the differential. The two components sit next to each other but have opposite chemistry needs. Stocking one 'universal' GL-5 for everything is the fast road to a returns problem.
How to choose for your customer
| Component | What to recommend | Why |
|---|---|---|
| Manual transmission (most passenger + light-commercial) | GL-4 (or maker's spec) | Brass synchros are vulnerable to GL-5 sulfur |
| Hypoid differential / final drive | GL-5 | High sliding load needs the EP level |
| Limited-slip differential | GL-5 LS (limited-slip variant) | Friction modifier for the clutch packs |
| Transaxle (combined) | Check the maker's spec — often GL-4 or a specific grade | One oil serves both; wrong choice damages synchros |
Viscosity grade and performance level are two separate axes
Importers sometimes conflate 75W-90 with GL-4 or GL-5. They are independent. '75W-90' is the SAE viscosity grade — how thick the oil is hot and cold. 'GL-4' or 'GL-5' is the performance level — how much EP load it handles. So a single viscosity like 75W-90 can be sold as GL-4 for manual boxes or GL-5 for diffs. When you build a gear-oil line, pick the viscosity your market needs first, then decide which GL level each SKU carries. BOLITCH supplies 75W-90 manual-transmission oil matched to the GL-4 specification used by most light-commercial and passenger manual gearboxes.
- 1Start from the vehicle maker's specification, not the number on a competitor's bottle.
- 2Lead with GL-4 for the manual-transmission and transaxle demand — that is where the volume and the mistake risk both sit.
- 3Add GL-5 (and GL-5 LS) for differentials and heavy axles once you see diff-related demand.
- 4Keep the two clearly labelled and never let a GL-5 sit on the shelf where a buyer might grab it for a manual box.
What to ask the supplier
- The API performance level (GL-4, GL-5) stated on the label and the TDS — and whether it is a true GL-4 or a 'GL-4 / GL-5' compromise blend.
- The copper-strip (yellow-metal) test result, so you can show buyers the GL-4 is safe for brass synchros.
- Base-oil type and whether the formulation is a sulfur-phosphorus EP system or a milder one.
- Batch traceability and the test report that ships with each lot, so the bottle matches the specification sheet.
Request gear-oil specifications
BOLITCH supplies 75W-90 manual-transmission oil matched to the GL-4 specification used by most light-commercial and passenger manual gearboxes, plus GL-5 for differentials. Tell us your market and vehicle mix, and we will send specification sheets, packaging options and quotes. Visit bolitch.com or email sale@bolitch.com. MSDS available on request.