What is the Difference Between Diesel Engine Oil and Gasoline Engine Oil?
3 Answers
Diesel engine oil and gasoline engine oil differ in heat resistance, viscosity, and additive requirements. Heat Resistance: Diesel engine oil is more heat-resistant than gasoline engine oil. The operating temperature in a diesel engine's crankcase is 20 degrees Celsius higher than that of a gasoline engine, and the piston crown and piston ring area can exceed 50 degrees Celsius higher. Typically, the piston crown temperature in a diesel engine ranges between 200-330 degrees Celsius, while gasoline engines do not exceed 200 degrees Celsius. Therefore, diesel engine oil is less prone to evaporation at high temperatures. Viscosity: Diesel engine oil has higher viscosity than gasoline engine oil. The compression ratio of a diesel engine is twice that of a gasoline engine, generally ranging from 12-22. The main components of a diesel engine endure much higher thermal and pressure impacts compared to a gasoline engine, with loads on the crankshaft bearings reaching 100-200 kg/cm². The extreme pressure can easily squeeze the oil out from between friction surfaces, making the viscosity and lubricity requirements for diesel engine oil stricter than those for gasoline engine oil. If gasoline engine oil is used, the oil film on the crankshaft and connecting rod bearings can easily rupture, leading to lubrication failure and dry friction. Additive Requirements: Diesel engine oil requires higher additive standards. Gasoline engines use Babbitt alloy for bearing materials due to its excellent corrosion resistance. However, diesel engines, which must handle higher bearing loads, temperatures, and faster sliding speeds between friction pairs, use high-performance materials like lead bronze or lead alloys for main and connecting rod bearings. These materials have poorer corrosion resistance, so diesel engine oil contains more anti-corrosion additives to enhance corrosion and wear resistance. Gasoline engine oil lacks these additives, and if used in a diesel engine, the bearings may develop spots, pitting, or even flaking, while the oil quickly becomes contaminated, leading to bearing seizure and other failures.
I've been driving trucks for over ten years, and the main difference between diesel and gasoline engine oils lies in their formulations. Diesel engine oil is designed to handle higher compression ratios in engines, containing higher concentrations of anti-wear agents and protective additives. API standards marked with a 'C' on the oil can indicate diesel engine oil, such as CI-4 and CK-4. Diesel oil has a much higher base number to neutralize the acidic byproducts of diesel combustion. Gasoline engine oils are labeled with grades like SN and SP. The biggest mistake is mixing them—using gasoline oil in a diesel engine will lead to insufficient anti-wear performance, causing piston ring issues quickly.
Last time I went for maintenance on my diesel pickup, the experienced mechanic at the repair shop pointed at the oil canister and gave me a lesson: The TBN (Total Base Number) of diesel engine oil is typically more than twice that of gasoline engine oil, specifically designed to combat sulfur compound corrosion in diesel engines. The most obvious difference can be seen in the viscosity rating—diesel oil packaging features labels like 'CJ-4,' and the back of the manual states 'suitable for diesel engines.' Nowadays, some so-called 'diesel/gasoline universal oils' are essentially diesel oil formulas with slightly lowered standards. Using them in gasoline vehicles is actually a waste of money.
