WHAT HAPPENS WHEN YOU MIX DIFFERENT ENGINE OILS?
Choosing the right engine oil is an important part of vehicle maintenance. Understanding viscosity grades, performance specifications and manufacturer approvals can help vehicle owners select an oil that meets their engine requirements.
Common Types of Engine Oil
Before discussing mixing, it is useful to understand three commonly marketed engine-oil types:
1. Mineral Engine Oil
Mineral engine oil is formulated primarily with mineral base oils produced through crude-oil refining. Its suitability depends on whether the finished lubricant meets the viscosity and performance requirements specified by the vehicle manufacturer.
2. Semi-Synthetic Engine Oil
Semi-synthetic, or synthetic-blend, engine oil combines mineral and synthetic base oils and is formulated to balance targeted performance characteristics and cost.
3. Fully Synthetic Engine Oil
Fully synthetic engine oils use selected base stocks and additive technology to meet defined performance targets across a range of operating conditions.
Key Factors to Consider Before Mixing Engine Oils
Engine oil is formulated from base oils and an additive system, which together influence the performance of the finished lubricant.
Engine oils contain carefully balanced additive systems that may include detergents, dispersants, antioxidants, anti-wear agents and other components. Finished oils are formulated to meet defined performance requirements, which may include API, ILSAC or ACEA categories and vehicle-manufacturer specifications or approvals.
Mixing Different Brands of Engine Oils
- Risk Factor: Mixing different engine oils combines formulations that may use different base oils and additive systems. Although the oils may physically mix, the resulting blend may not retain every performance characteristic or approval claimed for either original product.
- Specification and suitability: For a necessary top-up, use an engine oil that matches the viscosity grade, performance specification and any manufacturer approval required by the vehicle handbook. Using the same product is preferred whenever practical.
- Preventive Measure: If a substantial quantity of oil that does not meet the required specification has been added, consult the vehicle manufacturer or a qualified service provider regarding whether an earlier oil and filter change is necessary.
Consequences of Mixing Engine Oils
- Viscosity Variations: Mixing different viscosity grades changes the viscosity characteristics of the resulting oil. Without testing, the mixture should not be assumed to meet either original SAE grade or the vehicle manufacturer’s specified cold- and high-temperature performance requirements.
- Additive Balance: Lubricants are formulated with specific additive packages to enhance performance. Combining different formulations may change the concentration and balance of additives originally intended by the lubricant manufacturer. This may influence the oil’s ability to protect against wear, deposit, oxidation etc.
- Formulation Performance: The performance of a mixed oil cannot be determined solely from the individual product labels. Unless the combination has been evaluated, the mixture should not be assumed to retain all the performance claims of either original product.
- Operational Efficiency: If the resulting mixture does not meet the required viscosity or performance specification, characteristics such as fuel-economy performance and emissions-system compatibility may no longer be assured.
- Warranty and Compliance: Vehicle manufacturers specify oil requirements in the owner’s manual and service information. Using an oil that does not meet those requirements may affect warranty consideration where permitted by the applicable warranty terms.
Indicators You May Be Using the Wrong Engine Oil
The following conditions may be associated with lubrication-related issues; however, they can also result from unrelated mechanical or operating factors and should not be considered conclusive indicators of incorrect oil selection.
- Unusual Engine Noise: Ticking or other abnormal noise may warrant inspection of the oil level, oil pressure, lubricant suitability and the engine’s mechanical condition.
- Reduced Fuel Efficiency: A noticeable drop in mileage can sometimes be linked to oil viscosity or formulation differences.
- Excessive Exhaust Smoke: Variations in oil properties may affect combustion-related outcomes.
- Difficulty with Cold Starts: If the oil’s viscosity is too high for the climate, it can increase cranking resistance and feel sluggish on start-up.
- Frequent Oil Top-Ups: Oil consumption may be influenced by lubricant properties, but leakage, engine condition, operating conditions and mechanical faults should also be investigated.
Mixing may occur during an unplanned top-up, but the resulting blend should not automatically be assumed to retain the viscosity grade, performance claims or manufacturer approvals of either original product.
Always follow the vehicle manufacturer’s recommendation for SAE viscosity grade, performance specification and any required manufacturer approval. When a top-up is necessary, use the same product whenever practical.
Engine oils from different brands may have varying formulations and additive systems. When mixed, the resulting oil may not reflect the intended performance characteristics of either product individually.
While synthetic and mineral oils are generally compatible and their combination is essentially what creates a semi-synthetic blend—mixing them is usually only recommended as a temporary solution. Adding mineral oil to a high-performance engine that requires full synthetic can lower the overall protection level and thermal stability of the lubricant.
If you have used a different viscosity or brand for a minor top-up, the engine is typically safe to drive. However, to ensure the engine operates according to the manufacturer’s design, it is often suggested to perform a complete oil and filter change sooner than your next scheduled service to restore a consistent lubrication profile.
Mixing 5W-30 and 10W-30 engine oils will generally produce a lubricant with viscosity characteristics that fall between those of the two original oils. The resulting oil's flow properties at low and operating temperatures will depend on the proportion and formulation of each oil used.
