Can You Mix Synthetic Oil With Regular Oil?

If you use synthetic oil in your car and inadvertently top up with conventional oil, you might be concerned that you will damage the engine.

Other car owners think it is beneficial to mix synthetic oil with regular oil to improve performance.

Understanding the formulation of each oil and its uses provides some insight.

There are no cited issues with combining two types of oil nor will not cause any damage to the engine. However, what you might experience is reduced engine performance. Differing chemicals may interact and destabilize the formulation of the oils, causing the additives to be less effective. This will create further expense as you will need to drain and refill the tank with a single oil type.

Types of Car Oil

There are two main types of car oil, both of which are designed to lubricate all moving parts of a car engine and protect against friction and heat.

Traditional/Conventional Oil

This type of car oil is derived from natural crude oil that is drilled from pockets deep within the earth. It is derived from hydrocarbons and goes through multiple processes before it becomes the product that we recognize as car oil.

Crude oil goes to a refinery to have all waste products removed from it. This is called the sedimentation process and expels all of the impurities from the oil. It is then heated to an incredible 700F before being sent to fractionating towers to vaporize. The refining process is completed leaving a base oil.

This is the product that car oil manufacturers receive. They combine various other fluids to make their oil unique from other brands:

  • Detergents: Used to trap particles and break down coagulative sludge
  • Dispersant: Prevents the formation of large aggregate structures that result in sludge and varnish and might lead to phase separation.
  • Anti-wear agents: A chemical reaction forms a film barrier that prevents metal-on-metal contact and wears.
  • Rust inhibitors: Some form a physical barrier to seal out contaminants and water. Other types work by neutralizing harmful acids.
  • Friction modifiers: Improve the lubricity of the oil by making it more slippery. Better lubrication reduces friction and increases MPG.
  • Anti-oxidants: Reduce the harmful reaction that occurs when oil and oxygen combine, resulting in reduced sludge build-up.
  • Foam inhibitors: Reduce the surface tension of the air bubbles and causing them to collapse

Conventional motor oil is ideally suited for newer cars that are used for light-duty, commuting, and traveling short distances. The type of journeys that won't put huge demands on the engine and create high temperatures.

Synthetic Oil

Synthetic motor oil is made from a petroleum-based base oil combined with powder additives. A carrier oil ensures the even distribution of the additives.

It isn't an alternative, more a man-made replacement that will improve the performance of and protect the engine for longer.

There are many benefits to using synthetic oil:

  • Higher quality: Oil is more efficient and contains more consistent properties. It remains uniform and protects the engine across a very wide temperature range.
  • Fewer impurities This ensures it burns cleaner, thickens slower, which reduces engine drag.
  • Better fuel economy Better lubrication causes less friction enabling the engine to run with ease.
  • Longer distances between oil changes It works effectively over a wide range of extreme engine temperatures, reliably for thousands of miles.
  • Fewer emissions Mostly created by the burning off of engine gum. Synthetic oil is highly effective at preventing the build-up of gums and varnishes.
  • Quicker engine start time The viscosity and characteristics of synthetic oil are such that it provides smooth operation in all temperatures, including cold and icy weather.

Synthetic oil is suited for older cars that don't receive regular oil changes. Not only will it prevent new contaminants from forming, it can also help to clear existing sludgy deposits.

The main difference between conventional and synthetic oil is the quality of the base oil and the level of refinement it receives.

Can You "Top-Up" Oil Between Changes?

Adding oil between oil changes is a good cost-saving method. It might prolong the life of the existing oil, particularly if adding synthetic to regular oil.

Even if you're adding regular to a tank of synthetic, if the levels were significantly reduced, the engine would have been working harder causing decreased performance. Whichever way you do it, one oil will dilute the effectiveness of the other.

Mixed oil is available to buy. Manufacturers combine synthetic and regular oil to provide high-performance oil with a lower price tag than synthetic alone. Using this product is better than mixing yourself, the manufacturer will have ensured that the additives will combine and work together, not against each other and destabilize.

Switching between oils at consecutive oil changes won't have an adverse effect these days. It is thought to have created seal and gasket leaks on older cars, but modern technology ensures the two oils have enough similar properties to prevent this from happening.

When purchasing a pre-owned car it is not always possible to know which oil was previously used.

Which is the Best Car Oil

In almost all cases synthetic oil is the best product for your vehicle. It is higher-quality and doesn't contain the impurities found in regular oil.

It has a higher viscosity rating as it is much better at handling extreme temperatures, both high and low.

What is a Viscosity Rating?

viscosity rating

Viscosity changes as an engine heats from cold to very hot, oil thins, and the properties alter. High viscosity oil is thick and runs slowly, low viscosity is thin and runs faster.

There are a set of quantifiable characteristics of lubricants that the industry adheres to. Viscosity is one and describes and rates a fluid's resistance to flow.

Viscosity defines the force required to pass a 1-layer of fluid through another, at a given speed and temperature.

High-Viscosity Ratings

The rating is shown as 2 numbers which are achieved by testing the oil at 2 given temperatures, hot and cold, 0-degrees F. and 212-degrees F.

Oil with a 15W 40 viscosity rating acts as if it were #15 when cold and respond as #40 when hot.

5W 30 oil reacts as 5-weight in cold, wintry conditions, and 30-weight at regular engine operating temperatures.

Multi-viscosity oils provide the best protection for an engine through a range of extreme temperatures.

If you choose an inexpensive oil it should protect your engine well at 'normal' temperatures. Once the temperature goes beyond these parameters the oil will not behave as you would expect, it will become decreasingly effective and cause excessive wear and tear on the engine.

Should I Switch to Synthetic Oil?

Synthetic oil, unlike conventional oil, is made up of molecules of a consistent mass, size, and shape. This means that they create less friction as they collide, less friction = less heat.

The oil flows better in cold weather enabling reliable, cold weather starts and smoother running in winter.

Conventional oil breaks down or the molecules evaporate at extremely high temperatures. This causes metal-on-metal contact, creating friction, excessive heat resulting in damage and wear and tear. This doesn't happen with synthetic oil as it resists viscosity changes in high temperatures.

It is a slicker consistency, which ensures smoother and cooler engine operation. However, it should be noted that engines with existing leaks might mind the slickness of the oil exacerbates the leaks, resulting in burn-off and blow-by.

Synthetic oil is by far the most expensive option. This can be offset by longer intervals between oil changes; often vehicles can run for double the distance expected from conventional oil. Some industry experts suggest that a clean, modern engine run purely on synthetic oil, only requires a full drain at 25000-mile intervals.

Is Engine Oil the Same for Gasoline and Diesel Engines?

Diesel engine oil is more viscous and contains more zinc than gasoline oil. This is due to the different types of catalytic converter a diesel engine has. Diesel oil costs less and doesn't require changing as often.

Gasoline engine oil is less viscous, another reason the 2-types shouldn't be confused. If it inadvertently occurs you might notice engine deficiencies, particularly a loss in fuel efficiency. Once the mistake has been realized it is recommended to drain the tank immediately and replace it with the correct oil.

Gasoline engine oil is clearly marked with a starburst label reading API, American Petroleum Institute. It will also be marked 'S-type' (servicing) and 'C-type' commercial for diesel engines.

Final Thoughts

If it's possible, we recommend using synthetic oil to protect the engine of your car. It will enhance its performance, require fewer changes, and allow the vehicle to operate smoothly in all extreme temperature conditions.

There are going to be situations where sticking to the same oil isn't possible and you will need to know if you can mix synthetic oil and regular oil.

It is safe. If you add regular oil to synthetic oil you are going to dilute the high-performance properties of the synthetic oil. Adding synthetic oil to conventional oil won't noticeably enhance its performance.

If possible, use one type of oil at all times to protect the engine and ensure the car performs to the best of its abilities.

Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.
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