Your Most Common Motor Oil Questions Answered

motor oil

One thing the Internet has done is create a lot more “experts.” In the old days when you had a car maintenance question, you knew whom to ask – ‘ol Mike up the street with grease under his fingernails and a rag in his back pocket.

Nowadays, if you visit any online car forum, you’ll find dozens of purported experts whose only credentials are that they aren’t afraid to voice (or type) an opinion. Most of it is sound advice, and some of it, even when wrong, is pretty harmless. It’s safe to say, however, if you’re in doubt, check it out. Or, as the saying goes, “Fools rush in where angels fear to tread.”

All this to say that I’ve been asked a lot of questions over the years, even if I don’t have grease under my nails. I’ve been answering questions here at AMSOIL for more than two decades. One advantage I have over the average person is that I work with a lot of very smart people – chemists, engineers and wrench-turners – so that when I don’t know something, I do know whom to ask.

Here are a handful of questions I’ve heard a few times over the years. Maybe one of them was once yours.

QUESTION: I was cleaning my garage and found some old motor oil containers. Can I still use this oil in my new vehicle?

Ed’s Reply: I’ll try to answer based on the information you have given, which is incomplete. Was the oil synthetic or conventional?

A lot of people don’t realize that sediment can settle to the bottom of a can of old conventional oil. One time I took photos of a quart of oil I found at Walmart in which there was, indeed, sediment at the bottom. The bottle was clear, no doubt intended to show off the liquid gold within. Instead, it showed me how oil ages.

The real problem with older oil is that it may not meet current motor oil specifications. For example, the old oil might be labeled an API SJ, SL or SM. The current specification is API SN, which was introduced in 2011. API SF, SG and SH are obsolete. Recommendation: discard.

QUESTION: Can I switch to conventional oil after I have been using AMSOIL?

Ed’s Reply: Yes, you can. But why would you want to?

QUESTION: What’s the difference between diesel oil and regular motor oil used in cars that run on gasoline?

Ed’s Reply: All lubricants are the same in one respect. Each is produced by blending base oils and additives. The additives are used to combat destructive processes and enhance beneficial properties of the base oil. Diesel engines tend to produce more soot and acidic by-products, hence contain different additives. Gasoline engines face a different set of issues. I won’t bore you with details, but if you’re interested in the chemistry, there’s plenty more to making oil than just mixing a couple ingredients. Just as our world has become increasingly complicated and technical, so has engine technology. Fortunately there are a lot of smart people who work very hard to make sure the rest of us don’t have to worry about it.

the thinker statue amsoil hat(Want to know the difference between gas and diesel engines? Find out here.)

QUESTION: My uncle says synthetic oils make cars leak oil. He says something about seals that I don’t understand, but it made me afraid to switch to synthetics.

Ed’s Reply: No, synthetic oils do not cause engines to leak. If you have a mechanically sound engine (one that is not leaking), it won’t leak after you switch to synthetic. AMSOIL synthetic motor oil is fully compatible with modern seal materials and properly formulated to condition seals, keeping them pliable to prevent leakage.

QUESTION: My mechanic says synthetic oils may be better, but they are too expensive.

Ed’s Reply: Everyone is entitled to an opinion. Here’s mine. How much does your car insurance cost? Not cheap. How much does it protect your engine from wear? Zero. Does is produce an invisible shield around your car that keeps people from denting it? No. In short, you are paying money for nothing.*

When you use a premium synthetic oil like AMSOIL synthetic motor oil, it protects the engine from wear and premature damage. It shows, too, that you care about the car and, when you sell it, you’ll be able to get a premium price because you have demonstrated your consideration for detail.

Your mechanic is correct when he (or she) admits conventional oils are inferior. But the price gap has narrowed. In 1972 when AMSOIL synthetic motor oil was introduced, it cost $5.00 a quart while petroleum oil cost just $0.50 – ten times more!

Today, many synthetics are less than twice the cost of conventional oil. And the ones that cost more, like AMSOIL Signature Series Synthetic Motor Oil, provide a return on your investment in the form of better wear protection, improved engine cleanliness and longer oil life, making up for the higher initial price. Using Signature Series for its full 25,000-mile/one-year guaranteed drain interval costs less overall than multiple conventional oil changes.

Do you have more questions? Check out our FAQ Page, where we’ve answered a lot of questions over the years about all sorts of topics.

*EdNote: Please do not misunderstand my meaning. Insurance is a necessary evil, and required by law in most states.  


  1. Can BG MOA (Motor Oil Additive) be added to Amsoil Signature Series 5w-30? I know that Amsoil does not recommend adding any additives however this is a rhetorical question; what will actually and scientifically happen if MOA is added to the Signature Series?
    I have seen dealers add MOA to Amsoil and to Mobil 1.
    However, I do trust your opinion.

    1. Hi Husam,

      You’re correct; we don’t recommend using aftermarket oil additives with our products. Why? Because the oil is already finely tuned to provide all the benefits your engine needs, and adding an additive will only disrupt the oil’s chemistry and alter its performance. Simply put, you don’t need additives with our oil.

      What will happen? Given all the variables in play, we can’t definitively say without pulling an oil sample from your specific engine, adding the additive and running a bunch of tests. Obviously, that’s cost-prohibitive and filled with logistical challenges.

      Ultimately, it’s your engine and your decision. But please understand that adding aftermarket additives will void the AMSOIL product warranty.



  2. What’s the point of using AMSOIL 25,000 if it says OR 1 year, whichever comes first? If I drive 8000 in a year, it means there is no point to me getting the 25K oil because it’s 1 year and I am supposed to change it anyway, right? I mean that’s what AMSOIL SAYS. Now if they said I could drive it 25,000 over 2 years, that would entice me. Does anyone know what happens if you drive 25,000 over 2-3 years on one oil change? Is it not made for that? What if I have the good filter and only drive on pavement?

    1. Hi Mister Tumnus,

      Motor oil has a mileage limit and a time limit. While the former is self-explanatory, the latter requires some explanation.

      Even when you’re not driving, chemical forces work to degrade motor oil. The oil’s additives must combat combustion by-products and moisture in the oil, which reduce its Total Base Number (TBN). An oil’s TBN is a measurement of its ability to fight harmful acids. Once TBN becomes too low, the oil must be changed…even if it hasn’t accumulated that many miles.

      However, you can use Signature Series for longer than one year IF you use oil analysis. If your oil analysis report says the oil is still suitable for use, you can continue to use it regardless of how many miles and months (or years, potentially) are on the oil.

      So, why use Signature Series if you’re only driving about 8,000 miles per year? In short, because it’s our best oil when it comes to wear protection, engine cleanliness, turbo protection and the like. If you want the best for your vehicle, use Signature Series.

      If Signature Series isn’t for you, consider AMSOIL XL Synthetic Motor Oil, which is recommended for up to 12,000 miles or one year, whichever comes first. It delivers great protection at a mileage interval closer to what you drive.



    2. An excellent comment/question unless you have a long commute or you’re a hired ride service. Most people will not put on 25,000 miles per year and therefore the extra cost for such an oil would not be justifiable. I do not doubt that it is a high quality oil just not cost-effective for most. The reason it is rated for the mileage or the one year whichever occurs first is that the oil’s additive package will really be compromised after the one year timeframe, regardless of mileage. Additionally with these advancements in engine oil lasting so much longer now, don’t forget to still check your oil level on a regular basis. I have seen oil levels down a quart or so by the time the manufacturer’s recommended oil change interval was due.

  3. Just changed my oil and found the “oil” in the pan looked like peanut butter with similar consistency?? Any idea what’s going on? I’m faithful at changing it every 5000 mile. Have never seen anything like this???

  4. I have 2 new chevy vehicals one with a 5.3l flex the other with a 6.2l. Both under 20,000 miles. Is it healthy to go ahead and start running amsoil?

    1. Hi Steven,

      Yes, it’s perfectly “healthy” to go ahead and switch over to AMSOIL.


  5. I have a ford econoline with 150000 miles on it, should I switch to synthetic oil or not , I’m worried that with the higher millage , the old seals may start leaking with the thinner oil.

    1. Hi Peter,

      Ed is at the AMSOIL Engine Masters Challenge this week, so I’ll step in for him. If your engine is mechanically sound, meaning the seals don’t currently leak, then you’re perfectly fine switching to synthetic motor oil. You might want to flush the engine first. Check out this post for details on whether or not to flush. It’s a common misconception that synthetic oil is “thinner” than conventional oil and more easily slips past seals. This isn’t the case. A 5W-30 synthetic is the same viscosity as a 5W-30 conventional, not “thinner”.


  6. Should I change my oil in my motorcycle in fall after riding season and let it sit all winter? Or change it in spring and let the old oil sit in it all winter? Or can I drain the oil and let it sit empty all winter?

    1. Hi Larry,

      Since used motor oil contains acidic by-products, we recommend changing oil in the fall prior to storage. That way you have fresh oil protecting the engine from rust formation throughout the winter.

      Thanks for reading.

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