Engine oils are graded by SAE viscosity numbers. Among SAE 30, SAE 40, SAE 70, and SAE 80, which oil is the most viscous (thickest at operating temperature)?

Introduction / Context:
SAE viscosity grades indicate the resistance to flow of engine oils. Higher numbers generally mean thicker oil at the reference temperature. Selecting the correct grade ensures proper lubrication, film strength, and cold-start performance.

Given Data / Assumptions:

• Single-grade SAE numbers are being compared (no W winter designation here).
• All oils are evaluated at their standard test temperature for kinematic viscosity.

Concept / Approach:
For single-grade oils, a larger SAE number corresponds to a higher viscosity range at 100°C. Therefore, among the listed options, the largest number is the most viscous at operating temperature.

Step-by-Step Solution:

Verification / Alternative check:
Viscosity charts from standards bodies (e.g., SAE J300) show viscosity ranges increasing with grade number for non-W oils, confirming the conclusion.

Why Other Options Are Wrong:

• SAE 30, SAE 40, SAE 70: each has lower viscosity than SAE 80 at the specified test temperature.

Common Pitfalls:

• Confusing multigrade designations (e.g., 10W-40) with single grades; the logic still holds at operating temperatures for the second number.
• Assuming thicker is always better; excessively thick oil can reduce flow and efficiency.

Final Answer:
SAE 80