The principle fuels used in a modern grain dryer are natural gas, vaporized liquid propane,...

Fuel oil is a liquid fuel derived from petroleum that can be used in grain dryers as an alternative to natural gas or propane. Fuel oil is typically classified into six grades, from No. 1 to No. 6, based on their viscosity, sulfur content, and boiling point. No. 2 fuel oil is the most common grade used for heating purposes, including grain drying. Fuel oil has a higher energy density and lower cost than natural gas or propane, but it also produces more emissions and requires more storage and handling12

References:

Best Fuel for Grain Drying Equipment - Crown Oil1

Preventing Grain Dryer Fires: Tips For Startup, Cleaning, Shutdown2