Engine oils — which designation corresponds to a multigrade lubricant as per SAE viscosity grading (cold and hot performance indicated in a single label)?

Introduction / Context:
Lubricant viscosity affects cranking effort, oil pressure, hydrodynamic film thickness, and fuel economy. The Society of Automotive Engineers (SAE) defines viscosity grades. A multigrade oil combines low-temperature (W) and high-temperature viscosity performance in one formulation using viscosity index improvers, enabling year-round usability.

Given Data / Assumptions:

• SAE viscosity grades are of the form XW-Y for multigrades.
• API classifications (e.g., API SF) describe performance categories, not viscosity.
• Single-grade oils are labeled without the W/high-temp pair.

Concept / Approach:

A label like SAE 20 W-50 indicates that the oil meets cold-cranking and pumpability criteria for a 20W grade and high-temperature kinematic/dynamic viscosity limits for a 50 grade. This makes it a multigrade. In contrast, SAE 30 is a single-grade. API SF and “API 50” are not viscosity grades; the former is a service category (older gasoline engines) and the latter is not a standard API label.

Step-by-Step Solution:

Verification / Alternative check:

SAE J300 standard defines the limits for W and non-W grades; commercial data sheets for SAE 20W-50 oils show compliance with both low-temp CCS/MRV and high-temp viscosity requirements.

Why Other Options Are Wrong:

SAE 30 — single grade; no cold performance grade indicated.
API SF — performance/service classification, not viscosity.
API 50 — not a valid viscosity/service code in this context.

Common Pitfalls:

Confusing API/ACEA service categories with SAE viscosity; misreading the W as “winter weight” rather than low-temperature performance classification.

Final Answer:

SAE 20 W-50