For small, air-cooled, single or twin-cylinder diesel engine using splash lubrication in winter conditions, the core principle is: Prioritize cold-start performance while maintaining adequate high-temperature protection.
1. The Key Metric: Oil Viscosity Grade (e.g., 15W-40)
This is the most critical factor for winter.
· The number before the "W" (e.g., 0W, 5W, 10W): Indicates cold-temperature fluidity. A lower number means better flow at low temperatures, ensuring the oil can be splashed and pumped quickly during a cold start to prevent wear.
· The number after the "W" (e.g., 30, 40): Indicates viscosity at the engine's operating temperature (100°C). Air-cooled engines run hotter, so a sufficient viscosity (like 40) is needed to maintain a protective lubricating film under load.
Recommended Viscosity for Winter:
· Severe Cold (consistently below -15°C / 5°F): Use 5W-30 or 5W-40.
· Typical Cold Winter (approx. -10°C to 0°C / 14°F to 32°F): This is the most common and balanced choice.
· Mild Winter (above 0°C / 32°F): 15W-40 may suffice, but cold starting will be harder.
2. Essential Standard: Oil Quality (API Classification)
You must use oil specifically designed for diesel engines.
· Minimum Requirement: API CF-4 or higher.
· Better Choice: API CH-4, CI-4, or CJ-4. These higher grades offer superior detergency (to handle soot), oxidation stability (for high air-cooled temperatures), and anti-wear protection.
· Crucial: Oils starting with "S" (e.g., SN, SP) are for gasoline engines and are not suitable.
3. Oil Type: Mineral vs. Synthetic
· Mineral Oil: Cost-effective and sufficient for basic use if it meets the correct viscosity and API grade.
· Semi-Synthetic or Full Synthetic: Highly recommended for winter. They flow significantly better at low temperatures for easier starts and offer better high-temperature stability and longer drain intervals. A semi-synthetic diesel oil is an excellent value upgrade.
Summary & Direct Recommendation
For most users, the optimal balance is:
API CF-4 (or higher) 10W-40 Semi-Synthetic Diesel Engine Oil.
