These four terms get used loosely, but each one is formulated for a genuinely different job. Using the wrong category — even one with a "similar" viscosity — can cause real equipment damage.
Engine oil
Formulated for internal combustion engines — protecting pistons, bearings, camshafts, and valve trains from wear, while also cleaning (detergent additives), neutralizing acidic combustion byproducts, and helping seal piston rings. Engine oil sees extreme, rapidly changing temperatures and is graded by SAE viscosity (e.g. 20W-40) plus a performance rating (API, ACEA, or JASO — covered in our ratings guide).
Gear oil
Built for transmissions, differentials, and gearboxes, where gear teeth meet under very high, concentrated pressure. Gear oil carries a much heavier extreme-pressure (EP) additive package than engine oil, and uses its own SAE grading scale (like 85W-140) — a gear oil and an engine oil with the "same" number on the label are not interchangeable; the underlying formulation and additive chemistry are completely different.
Hydraulic oil
Powers hydraulic systems — excavators, presses, lifts — by transmitting force through pressurized fluid rather than protecting metal-on-metal contact the way engine or gear oil does. Hydraulic oil is graded by ISO VG (Viscosity Grade) rather than SAE, and prioritizes clean, stable flow, anti-wear protection for pumps and valves, and resistance to foaming and oxidation over long service intervals.
Grease
Not a liquid at all — grease is oil combined with a thickener (commonly a metal soap, like lithium or calcium) to hold it in place at a bearing, joint, or fitting where a flowing liquid would simply leak or drip away. Grease is graded by NLGI consistency number, not viscosity — see our dedicated grease guide for the full explanation.
The practical takeaway
Match the product to the job, not just the number on the label. If you're unsure which category a piece of equipment actually needs, our Product Matcher can help — or reach out directly and we'll confirm before quoting.
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