What Engine Oil Should I Use?

The first thing you need to look for is that the oil is made for a diesel engine.

Then you need to pick the correct viscosity. The oil viscosity (the numbers on the jug) need to match the climate that you are working in as closely as possible. The numbers, such as 10W-30 or 15W-40 indicate the oil's viscosity or thickness.

The first number before the 'W' (which stands for “winter”) indicates the oil’s viscosity at 0 degrees Fahrenheit. The second number indicates the oil’s viscosity at 212 degrees Fahrenheit. The larger the numbers, the higher the viscosity. Higher viscosity oils are heavier and typically coat the engine better to provide more lubrication and protection but can get too thick in colder temperatures. Lower viscosity oils work better in cold weather and also provide improved fuel efficiency but can become too thin to properly coat and protect engine components in hotter climates.

The solution... chemistry. Oil manufacturers developed additives that can be put in the multi-weight oil to allow for a lower viscosity to help with an easier cold startup but then also allow the oil to perform at a higher viscosity as the oil heats up during operation so that it does not get too thin. 

The short answer: The typical weight oil for Diesel Yanmar tractors in most areas of the USA is going to be a 10W30 but warmer areas, especially during the summer, might benefit from a 15W40 oil. Check the back of the jug to be sure you are getting a weight that is formulated for your area.