Formulation Guide

Engine Oil Technology Explained — SAE Grades, Viscosity, API Specs and Base Oils

Behind every grade on a bottle — 5W-30, 15W-40, 0W-20 — is a system of viscosity ratings, service specifications, base oils and additives. This guide explains what the SAE grade means, how API/ACEA/BIS specifications work, the difference between mineral and synthetic, and how an engine oil is put together.

In short: an engine oil is 75–90% base oil plus 10–25% additives. The SAE grade (e.g. 5W-30) sets its viscosity behaviour cold and hot; the API/ACEA/BIS specification sets the performance it must deliver; the base oil group and additive package are how a formulator hits both. Multigrade oils exist because a viscosity index improver keeps viscosity stable across temperature. Engine oil is one product of the wider lubricant formulation discipline.

The four building blocks

SAE viscosity grades

The SAE J300 system rates oil in two parts. The "W" (winter) number rates cold-crank and cold-pumping flow; the second number rates viscosity at 100 °C plus a high-shear (HTHS) minimum. A single-grade oil meets one; a multigrade meets both — made possible by a viscosity index improver. How VII enables multigrade oils →

API, ACEA & BIS specifications

Engine oils are certified to service categories: API "S" (petrol) and "C" (diesel), the European ACEA sequences, and — in India — BIS IS 13656. These define the tests an oil must pass, which in turn set the additive package and base-oil quality. BIS IS 13656 certification →

Base oils — mineral vs synthetic

The base oil is 75–90% of the finished oil. Group I/II are mineral; Group III is hydrocracked (often marketed "synthetic"); PAO and esters are true synthetics. Higher groups give better oxidation stability and low-temperature flow, enabling grades like 0W-20. Synthetic ester base oils →

The additive package

On top of the base oil sits 10–25% additives — detergents, dispersants, anti-wear (ZDDP), antioxidants, VII and PPD — balanced against SAPS limits. This is what makes an oil pass its target specification. Engine oil DI package →

How to read an SAE grade

Example	Winter (W) part	Hot part (100 °C)
0W-20	0W — excellent cold-start flow	20 — thin, fuel-economy
5W-30	5W — very good cold flow	30 — balanced modern PCMO
15W-40	15W — moderate cold flow	40 — heavy-duty diesel workhorse
20W-50	20W — warm-climate start	50 — thick, high-ambient / older engines

For the full detail, see the deep-dive guides on SAE viscosity grades (the exact J300 cold and hot limits) and API engine oil categories (SP, CK-4, FA-4). Choosing and hitting a grade against its API/BIS target is the work of lubricant oil formulation and ASTM testing.

Explore engine oil formulas by grade

Frequently asked questions

What does 5W-30 mean?

The number before the W is the winter (cold) rating — lower means better cold-start flow. The number after is the high-temperature grade, measured at 100 °C. So 5W-30 flows well when cold and holds a 30-grade film when hot. This dual rating is what makes it a multigrade oil.

What is the difference between 5W-30 and 5W-40?

Both have the same 5W cold-start behaviour. The 5W-40 is thicker at operating temperature (higher HTHS viscosity), giving a stronger film for high-output or hotter engines; 5W-30 is thinner for better fuel economy. Always follow the engine maker’s specification.

Is synthetic oil better than mineral oil?

Synthetic base oils (PAO, esters, Group III) offer better oxidation stability, low-temperature flow and high-temperature protection than Group I/II mineral oils, which is why they enable wider grades like 0W-20 and longer drain intervals. Semi-synthetic blends balance cost and performance.

What does API SP or CK-4 mean?

API service categories certify performance. "S" categories (e.g. SP, SN) are for petrol/gasoline engines; "C" categories (e.g. CK-4, CI-4) are for diesel. The later the second letter, the newer and more demanding the standard.

What viscosity grade should I use?

Use the grade specified in the vehicle owner’s manual for the operating climate. Going too thin can reduce film protection; too thick can hurt fuel economy and cold starts. For a new product, the target grade is set by the OEM/BIS specification you are formulating to.

Developing an
engine oil to spec?

From base-oil and additive selection to hitting API/ACEA/BIS pass criteria with a full BOM and test data — Lubechem formulates and validates the finished oil.

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