Which Engine Oil is the “Perfect Oil”?
Motor oil is the most important component of your engine. Without it, your engine dies. With the wrong oil, your engine dies slowly. Getting it right really does matter. Use this guide to help you determine which engine oil is the best selection possible so that you can get as much life as possible out of your engine.
There is NO Perfect Oil
Before we start, you need to know there is no "perfect oil" for every circumstance. There isn't even a "perfect BRAND" for all situations. Everyone's situation is different. Their driving habits are different. Their goals are different. Their vehicle's are different. As a result, the perfect oil for you won't necessarily be the perfect oil for your neighbor or your brother-in-law.
People get pretty attached to particular oils and especially particular brands. And, they'll often spend alot of time trying to convince someone that the answer to the "Which Engine Oil" question is most definitely BRAND X or BRAND Y. Unfortunately, more often than not, they are wrong.
Just know that the "perfect oil" for you is the one that meets YOUR specific needs, and this article is designed to help you determine what those needs are and how to best address them.
Which Motor Oil Type?
First and foremost, in deciding which engine oil you should purchase, do not overlook the fact that viscosity is not the only criteria critical to proper motor oil selection. You must choose an oil that is matched to your application with regards to it's formulation and design, specifically basestocks and additives used. For example, if you are running a diesel engine, then you had better be sure the oil you choose is rated for diesel engines. Similarly, if you are running a gas engine, then the oil should be formulated for use in gasoline engines.
Of course, sometimes, an oil can be rated for both gas and diesel use. However, your oil should meet at least the minimum API specifications required for YOUR vehicle. For instance, if your vehicle manual calls for an API SL oil (which is a gas engine oil specification), then your oil should specify that it meets/exceeds that minimum standarad (in other words, is should be rated for API SL, SM, SN, ...). It MAY meet other specifications as well, but it MUST, at a minimum, meet the API SL specification.
Likewise, if you need to know which engine oil to run in your diesel engine and your owner's manual specifies that you should only use API CI-4 oils, then you should not use an oil that is only rated for CH-4. This is not a recent enough specification. However, if the oil you are considering indicates that it meets API CJ specs, then you're ok, since "J" comes after "I". API specs are generally "backwards compatible", meaning that the CJ spec also is appropriate for use in vehicles calling for CI, CH, CG, CF, etc. (all PREVIOUS API classifications).
"Which Engine Oil" is Sometimes OEM Specific
Moreover, some vehicle manufacturers will specify OEM specific ratings that an oil must also meet. This is more common with European vehicles, but is becoming more common with US manufacturers as well. So, if your owner's manual says that your oil is to meet BMW specification XYZ123, then you had better make sure that the oil you're considering indicates that it meets that OEM spec.
For more information regarding API and other motor oil classifications and how they are determined and used, see the following article: Understanding Motor Oil Classification Systems.
Which Basestock: Synthetic or Petroleum
This is a question that is actually a relatively critical one IF you have any concern over how long your vehicle/engine should last. There are plenty of situations that do NOT warrant the use of synthetic oil, and you may very well be in one of those situations, depending upon your vehicle, how you drive, where you drive or how long you intend to keep your vehicle. Of course, it is just as likely that your situation may very well warrant the use of synthetic oil, and the decision regarding which engine oil basestock you should choose is not necessarily a simple one.
The truth is, there are MANY variables that affect this decision, not the least of which is just how much trust you put in a synthetic oil to offer enhanced protection and possibly extended oil drains. Some of the many benefits that synthetic oils offer can only be taken full advantage of if you actually trust them to offer those benefits and push the oils beyond what a conventional petroleum oil could accomplish.
The true differences between synthetic oils and petroleum oils and the discussion of when and why you might choose one over the other is one that truly is beyond the scope of this article. If you really want to delve more into this question, I highly recommend you consider downloading my eBook "The Motor Oil Bible". You can get the download here.
The Viscosity Question
In most cases you can simply go with the manufacturer's recommended viscosity grade. Of course, if you're considering a synthetic oil, it is generally very safe to go to a lesser "w" rating than what is recommended by the manufacturer (although the higher the quality of the synthetic, the more true that statement is). For instance, if the manufacturer recommends a 10w30 viscosity (and isn't already specifying the use of synthetic oil), then you would generally be safe to use a 5w30 or 0w30 synthetic oil.
Generally (see below for exceptions), if the vehicle manufacturer specifies an XwY viscosity oil (such as 5w30, where "X" is "5" and "Y" is "30"), under most circumstances you should stick with the same "Y" value, but can go to a lesser "X" value to improve cold weather performance and possibly fuel efficiency. However, in my opinion, this is ONLY true if you're moving to a synthetic oil and synthetic is not already being specified by the vehicle manufacturer.
In other words, if your vehicle manufacturer is calling for a 10w30 SYNTHETIC oil, then, although you MAY be able to go to a 5w30 synthetic oil, I, personally, would not recommend it without some significant investigation into the "which engine oil viscosity" question. On the other hand, in nearly all cases, if the vehicle manufacturer is calling for a 10w30 oil and does NOT specifically indicate that you should use a synthetic oil, then, you should be able to EASILY move to a 5w30 synthetic oil, and possibly even a 0w30 synthetic oil with no trouble at all.
If the manufacturer specifies multiple viscosities that can be used in your vehicle, but the manufacturer does NOT specify the need to use synthetic oil, then a petroleum oil of ANY of those listed viscosities would be fine, although, the conditions under which the vehicle will be driven would dictate whether to stay at the "high end" or "low end" of the viscosity range listed.
Which engine oil viscosity you choose will be affected by the answers to these (and possibly other) questions:
Will you be driving in cold temperatures mostly? Congested city driving where engine temps can rise considerably? Heavy towing (same heat issues)? Do you live in a very hot climate? All of these and other driving conditions should be taken into account in trying to find the BEST viscosity for your application. However, ANY of the viscosities listed within your manual should be adequate.
In contrast, if the vehicle OEM is not specifically calling for synthetic, but you're planning on going that direction, then, nearly always, you can safely stick to the lowest "Y" value listed and could even go to a LOWER "X" value. So, for instance, if the manufacturer said you could use 10w30 or 10w40 in your vehicle (and doesn't specifically call for synthetic), unless your application falls under one of the exceptions below, you could safely stick to an 5w30 oil (the lowest acceptable "Y" value listed in your manual) and you could go with a 0w30 or 5w30 oil safely, even though the manufacturer listed only 10w options.
Exceptions to the Above Rule
1) If yours is a racing application, more investigation would be necessary to determine which engine oil viscosity to be using in your engine
2) If yours is a heavy duty application where your vehicle is being subjected to EXTREME service such as towing extremely heavy loads, you may want to consider going to one of the heavier recommended viscosities.
3) If you are not comfortable venturing outside of OEM viscosity recommendations, then, by all means stick with what your vehicle manufacturer recommends. OEM viscosity recommendations will be perfectly safe for your vehicle and will make your selection process easy.
It is possible, depending upon how you are using your vehicles or equipment, that a more extreme divergence from the OEM viscosity recommendation might be appropriate. However, if you think this may be the case, before making the decision to diverge entirely from the manufacturer's viscosity recommendation, please do your homework.
For more detailed information related to the question of which viscosity to use, see the following article: Which Engine Oil Viscosity?
Motor Oil Quality Comparisons
Once you've managed to decide the type and viscosity of oil you should probably be using, the question then comes down to determining which of the oils that meet your criteria are the best of the bunch. In most respects, the only way to really make that determination is to check the specs. It's not a perfect technique, since the specs can "lie" in some ways, but, it's the best option you've got. In order to help you with those comparisons, I've implemented a free online oil comparison database that will allow you to quickly and easily compare the specifications for over 1000 different motor oils.
Search, sort and filter the list any way you like to investigate only those oils that meet your specific criteria. Then, take a closer look at the top 3 or 4 oils on the list and make the best educated decision you can. You can check out this free oil comparison service here.
Much, Much More Information
If you really want to know all that you can about motor oil and filtration and which engine oil you should be using in your vehicle, without getting bogged down with super technical terminology, consider downloading the full 350 page version of "The Motor Oil Bible" eBook. I guarantee that by the time you finish reading it, you'll know more about motor oil and filtration than 99% of the people you will EVER run into. Get it now.