Why Some Engine Oils Are So Much Better Than Others – The Differences Will Surprise You.

There’s a lot of religion and dogma about motor oils – both on the street and on the track. We thought we would apply some science to it. We’ll explain why it’s okay to change your oil every 10,000 miles with some oils and every 5,000 with others. The differences will surprise you. We’ll deep dive into the differences.

Here’s a quick summary:

  • Don’t believe the marketing of oil companies. Talk to an engine rebuilder. Often, companies that market the most have the worst oil.
  • Conventional oils vary widely. Some are terrible, some are good. Pay 5 or 10% more and get decent conventional oil that will last at least twice as long, or longer.
  • Don’t switch from conventional to full synthetic on an older car, it will leak – a lot.
  • If you have a turbo or supercharger, always use full synthetic.

Okay let’s dive in:

There are three basic types of oil on the market.

  1. Conventional
  2. Semi-synthetic
  3. Full-synthetic

So what does this mean? Let’s go through each:

Conventional: It’s made from oil that comes out of the ground. It’s natural and since nature doesn’t usually make anything pure, it has a whole lot of other stuff in it. The “stuff” depends on where it was drilled (for example Texas versus Canada) and how much of it was “cleaned” or refined. Most people also add some other ingredients, giving marketing something to talk about.

Synthetic: This doesn’t come from the ground, it’s made in a factory. This means that it comes out very pure. Most vendors will add good stuff back depending on the vendor and the application (for example street or race use, etc.).

Semi-synthetic: This is just a mix of both.

How do they vary among the brands?

Some brands are more refined than others (making them more pure). They also have different additives or different amounts of additives. The differences make a significant impact on the way the oil behaves with time and heat.

It’s kind of like saying boiled chicken feet and premium fried chicken are the same because they come from a chicken.

They’re both from a chicken, so they should be the same right?

The quality of the oil determines how long this process takes or how much heat is necessary. We should emphasize that heat dramatically accelerates the process, but it can happen with time alone. Conventional oil that sits in a stored engine for decades is just like driving for 30,000 miles without an oil change.As background, all oils breakdown with heat and time. Eventually, the oil starts to change thickness (or viscosity). It will begin to get clumpy, start to burn and leave a coating on everything.

When you pull apart an engine that’s had super cheap oil changes at the manufacturer’s recommended interval (normally 6,000 miles or more), you can instantly tell. Some engines will have clumps that feel like canned mushrooms in your oil (gross!). In spots, it will burn and deposit a thick coating on parts (think about the bottom of a pot of chili that was left on the stove too long). Clumps or solids are REALLY bad for the engine. The engine needs oil at the proper thickness to function. If it’s clumpy, it won’t flow to all the parts. Small passages might get blocked entirely.

Some gross, clumpy oil that is obviously past due for a change. Source: Quora (link)

Better performing conventional oils do the same, but break down at much higher temperatures and over longer periods of time.

The difference in performance between good conventional oil and poor conventional oil is dramatic. If you’re pulling apart a motor, it’s hard to tell if an engine had good conventional oil or full synthetic (assuming oil changes at 6,000 miles or less and no turbo or supercharger – more on that later).

Also, remember the old myth that states you should change your oil every 3,000 miles? Even the worst oil will last 3,000 miles in your average car. But why not spend a few dollars more and change the oil half as often?

What about synthetic oils?

Synthetic oils are really “stable” compounds. That means they take a lot of heat to break down or burn. As I recall, synthetic oils must get almost twice as hot to burn compared to conventional oils.

What does that mean? Some cars have engines that run really hot such as race cars, powerboats, or anything with a turbo and supercharger (especially if they are oil cooled superchargers or turbos). These cars really need full synthetic oil. Anything less will burn up and clog the oil system. Ever hear of a turbo that died with very few miles on it? Chances are cheap oil had something to do with it.

In most engines, the oil never gets hot enough to start burning high-end conventional oils. Therefore, the engine doesn’t know the difference between good quality conventional and synthetic oils – assuming reasonable (less than 10,000 mile) oil changes.

So how often do you have to change synthetic oil?

Marketers will have you believe it’s up to 15,000 miles or more. While that might be true for some engines, heat and chemical breakdown normally isn’t the limiting factor for synthetic oils. The limiting factor becomes contaminates.

Not everything burns in the cylinder. Some stuff gets left behind and can end up in your oil. The oil will absorb some amount of this “stuff” – like of like water will absorb salt up to a point. The amount of “stuff” oil can absorb is called “solubility”. With enough miles, the oil becomes so dirty that it needs to be changed. The oil can’t hold any more “stuff”. The stuff is also generally too fine to be caught by the filter.

Newer engines are much better at burning everything and leave less “stuff”. So while I wouldn’t go 15,000 miles between full synthetic oil changes in a 1964 Caddilac, I might consider it in a new hot hatchback.

One more important thing: Synthetic oils also have a low “solubility” when compared with conventional oils. That means they can hold less dirt. Diesel engines are very dirty. Because of the dirt, they are better off with a conventional oil that will absorb more dirt.

Should I switch to synthetic oils?

Here’s the interesting part. Conventional oils have all sorts of nasty stuff in them (some more than others). That nasty stuff makes your engine seals swell. If you switch to full synthetic, all the seals will probably shrink. Your car will leak from places you didn’t know it had. So, we wouldn’t recommend changing unless the car has few miles on it. If you’ve made this mistake, that’s okay just switch back and the seals will swell again.

What about blends or semi-synthetic?

This is hard to discuss. We haven’t seen enough engines to know whether the oil they’re selling is 1-part boiled feet and 10 parts premium Church’s fried chicken or the other way around. We suspect it’s probably 10 parts feet and 1-part Church’s.

What’s the difference between race oil (AMSoil makes some) and full synthetic street oil

Street oils must conform with emissions laws. In other words, they can’t put anything they want into the oil (for example lead) that will burn off and cause problems. For street use, there are some common industry specifications that oils must conform too. For race oil, the door is wide open (and often your pocketbook). These additives often increase temperature tolerance of oil and sometimes leave an anti-wear coating on parts. The difference can be tiny, if any, between the two. Plenty of race teams use street oil.

So, what do we recommend?

Thanks to attorneys, I can’t tell you which oils are terrible. I can tell you that Castrol GTX is a good conventional oil for daily drivers without turbos. There’s a reason why you see it called out by name on the refill cap of some high-end cars. I use Mobile1 for full synthetic when it’s needed, but I haven’t tested a lot of other oils. I stuck with Mobile1 after I asked an Indy car mechanic which oil they used. Their response was, “because we’re sponsored by an oil company [other than Mobile], we can’t tell you that we use anything else … but I can tell you that we have plenty of Mobile1 around the shop. We’ve found that it’s the only oil that holds up.” For old cars (60’s era), I use a zinc additive to keep my flat tappet cams from wearing – a problem that popped up after the removal of lead in gasoline.

In our customer’s cars, we never allow mechanics to use anything but quality oil – just like other any replacement part.

Stay tuned. In future posts, we’ll talk about what picking the right viscosity (in some cases ignoring the manufacturer), filters, and gas mean.

3 replies
  1. Raj Patel
    Raj Patel says:

    Very useful information with type of oil and the impact on the engine. I always believed that synthetic oil was always good for old or new cars and that for turbo you should only use synthetic oil.


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