[ Viscosity | Choosing | Conventional Oil | Synthetic Oil | Synthetic Blends | Oil Additives | Links ]
OK. Here we go. I decided that I would attempt to enter the realm of engine oil. Great battles have been fought here, and I hope not to start another engine oil holy war. I will attempt to present the facts as straight as possible without interjecting my opinion too much, but a lot of these choices are a matter of opinion.
No matter what type of oil you choose, there are a few universal constants about engine oil. The first is change it regularly! Conventional oil must be changed with regular frequency (usually every 3,000 to 5,000 miles). The fact is that the oil simply breaks down as it ages and loses the ability to lubricate. It also collects contaminants from the combusting air and fuel in the engine. As these contaminants build up, the oil can become acidic and damage bearing surfaces. Synthetic oil does not breakdown as quickly, but it still acquires contaminants the same way. However, it has additives to help battle this. Change it at least every 5,000 - 6,000 miles unless it is specifically designed for long duration use. Many replace it at 3,000 miles just to be safe. Always change your oil filter when you change your oil. It does little good to replace your oil, only to have a half a quart of old oil sitting in a plugged filter.
It's also a good idea not to mix brands of oil. Stick with a particular brand at least for your current oil change and when adding oil later, keep using the same brand. Always have an extra quart or two readily available. Although you can change brands between changes, I recommend choosing a brand and sticking with it to avoid potential incompatibilities.
Always use standard conventional 5W-30 or 10W-30 oil when breaking in a new or rebuilt engine. Never use synthetics or additives. In fact, some of the new conventional oils have been causing problems in rebuilds of older engines because of the new friction inhibitors now available. I like to use the cheapest oil I can find (generic branded) for the break-in. The new engine parts depend on the characteristics of older conventional oil to properly break-in. A certain amount of friction during break-in is required or the new component surfaces will not wear to match one another, which inhibits the ability of the oil to form the film that keeps metal parts from touching one another. It also reduces accuracy of the tight tolerances needed in places like the piston rings. The result can be piston rings that won't seat or even spun bearings.
The way that oil viscosity is referred to is by its weight. In this case, "weight" refers to the thickness of the oil and is usually measured with a Zahn cup. Any liquid has some viscosity and that viscosity changes depending on its temperature. Usually, viscosity (thickness) decreases as temperature increases. This also is true of engine oil. Engines need a thin oil at startup, so that it can get to the engine components quickly, but it needs a thicker oil when the engine is hot because a thin oil becomes too thin. This is why engine oils are supplied as dual-grade weights. When you see 10W-30 on a quart of oil, it means that it acts like 10 weight oil when it is cold (the "W" means winter, say -10^C), but acts like 30 weight oil when hot (100^C). This is not to say that it is actually thicker when it is hot. Hot 30 weight oil is thinner than cold 10 weight oil. Even so, it still helps provide the benefits of both types of oil depending on its temperature. So 0W-30 oil acts like 0 weight oil when cold, but maintains a 30 weight viscosity when hot. Think of it this way: when your engine is hot, there is basically no difference between 0W-30, 5W-30, and 10W-30 oil. They are all acting like 30 weight oil at this point. It's at cold startup, when almost all engine wear occurs, that the viscosity is different. The 0 weight oil will get to the engine components quicker than the 10 weight oil, but in reality cold 0 weight oil is still thicker than hot 30 weight oil. On the other hand when the engine is cold, there is no difference between 10W-30 and 10W-40 oil. However when the engine is hot, the 10W-40 oil is thicker than the 10W-30. This is why single-grade oils are very bad. Straight 30 weight oil is way too thick when cold to properly lubricate the engine. The only way to use single weight oil is to have an oil pan heater to bring the oil up to operating temperature (about 140^F or 60^C) before the engine is ever started. If you simply must use it, this type of oil should only be used in race engines with pan heaters.
As far as what oil viscosity is best, it depends who you ask. The fact is that engine oil maintains its viscosity better than ever and synthetics maintain it the best. In my opinion, the best viscosity for all weather is 5W-30. The reason is that both 5W-30 and 10W-30 breakdown at about the same point. Lighter oil can get into places that the heavier oil cannot and will get there more quickly, even when cold, so why go heavier? The only reason would be leaks. If you have oil leaks, a heavy oil will go through them more slowly. That is why you can buy "No-Smoke", which is basically sludge to thicken your oil. If your engine is sealed well, feel free to try Mobil 1's 0W-30 oil. If that "0" really makes you nervous, stick with 5W-30. 10W-30 is unnecessarily heavy when cold.
Choosing an oil can be confusing because everyone has an opinion. I have one too and it has changed as the evidence had showed itself in our driveway. Here is a comparison of four cars using various types of oil and oil changing methods. All four were driven in similar conditions by two people (my father and myself) with very similar driving styles. The last one is for an N/A engine, but it makes a point.
At 120,000 miles, our 1987 Daytona Turbo I got a new camshaft and followers because the old ones were shot. Also, the bores had a noticeable ridge on them from ring wear. This engine got whichever name-brand conventional oil was on sale every 5,000 - 6,000 miles.
At 100,000 miles, I had my 1987 Plymouth Sundance Turbo I apart and the cam and bores had some wear on them. The bores had about the same wear as the Daytona, and the cam was worn, but in better shape. This engine got whatever name-brand conventional oil was on sale every 3,000 - 5,000 miles (5W-30 in the winter and 10W-30 in the summer).
At 110,000 miles, I had my 1987 Dodge Shadow Turbo I apart and the cam and bores had almost no wear on them at all. There was some coking of the oil in the oil supply line to the turbo. This engine always got Pennzoil conventional oil every 3,000 miles (5W-30).
At 104,000 miles, I had my 1987 Shelby CSX Turbo II apart. The bores had NO wear on them at all; I could not see or feel a ridge of any kind and the bearings were in excellent shape. The cam was a roller cam, so it naturally had no wear (not a fair comparison). The inside of the engine was clean and the oil lines were perfectly clear. This engine got 5W-30 Mobil 1 synthetic oil every 3,000 - 5,000 miles.
At 240,000 miles (not a typo), my uncle had the head off of his 1985 Pontiac Firebird (2.6L V6). This engine still looked new. The bores and cam were in excellent shape and the engine runs as quiet as a mouse. It got synthetic oil every 3,000 - 4,000 miles. By 320,000 miles (yes, really), he finally decided to replace the engine with a new long block. The original was starting to get some rod knock, but was still using very little oil. That car is still running and is on it's 4th transmission....
Some tests have been done using several different brands and types of oils in vehicles. Measurements were made of engine parts to begin with, then the vehicles were used in a similar way for several months, then taken apart and measured again. In my opinion, these results are inconclusive because the length of time was too short. The amount of wear was found to be the same, but since the tests were only conducted for a few months, the wear found was primarily break-in wear. A similar test lasting several years and tens of thousands of miles is needed to make any real conclusions.
So basically, the only advice I can give about conventional oil is to buy a well-known name brand oil. Whatever brand you choose, I highly recommend that you do not mix brands of engine oil. Chances are that nothing will happen, but there is the possibility that the additives of one brand can react with the additives of another. Changing brands between oil changes is probably fine, but don't use a different brand if you have to add oil later. Some brand suggestions (in alphabetical order) that I have used in the past are: Castrol, Havoline, Mobil, Pennzoil, Quaker State, Shell, and Valvoline.
One thing to remember is to always use conventional engine oil with no additives (Slick 50, Prolong, etc.) after an engine rebuild. The new engine parts depend on the characteristics of conventional oil to properly break in. A certain amount of friction during break-in is required or the new components may fail to break-in.
Synthetic oil is also put together the same way as conventional oil. It has a base and additives. The difference is that the base oil is synthesized so that the size of the molecules are ideal for a particular weight and are of consistent size. Conventional oil has many molecule sizes all mixed together with many impurities, but there is an ideal size for the best lubricative properties and viscosity. With a pure base, there are no waxes or impurities that contribute to buildup of "varnish" and "coke" in your engine. A more sophisticated set of additives is added to this ideal base oil. These additives make for an extremely stable engine oil which can maintain its viscosity over a larger temperature range and keeps the base oil molecules from breaking down. The result is an oil that can flow at much lower temperatures, maintain proper viscosity at higher temperatures (thermal breakdown), and remain stable for a much longer period. It will also protect your engine longer at and right after startup because the friction inhibitor additives are activated a lower temperatures than that of conventional oils. Probably the two most popular brands for synthetic engine oil are Mobil 1 and Castrol Formula SLX (formerly "Syntech"), though other brands are available. Mobil and Castrol synthetic oils are compatible with conventional oils and can be mixed with them. If you are going to blend synthetic with conventional, I highly recommend using the same brand of both oils to ensure compatibility.
The only real downside to synthetic oils is the cost. It is typically two or three times the cost of conventional oil for a good synthetic. Another little-known downside is that switching to a synthetic oil on an old engine can result in oil leaks. Why? Because the detergents in the synthetic oils will "clean-up" the varnish and sludge left by conventional oils. If your engine seals are worn, the synthetic will break down the oil varnish that may be maintaining the seal. So it's not that the synthetic oil caused a leak, it just that it revealed worn seals by cleaning the varnish off of them. Many people have switched to synthetic on 100,000+ mile engines with no leaks, so it just depends on how often you changed your oil and the overall condition of your engine.
Conventional oils are sufficient for most engines, but I personally recommend synthetics for turbocharged and other high-performance engines. Oil running through a turbocharger has the undesirable job of removing the extreme heat from the turbo's bearings and shaft. It is not uncommon to see oil coking in the oil supply line to the turbo when conventional oils are used, even when they are changed frequently (though that does help). Synthetic oils will not do this. Also, I have seen many engines that have run only Mobil 1 and are in amazing condition. The evidence is in our driveway (see top pf page).
Blended oil is an alternative to buying pure synthetic. It combines some synthetic base oil with conventional base oil. Typically it does not include the synthetic oil's advanced additives. The small percentage of synthetic base oil in a blend versus its price does not make it worthwhile, however. If you want to run a blend of oil to save money, I recommend blending the oil yourself. Put one quart of the synthetic oil of your choice and fill the rest with the same brand of conventional oil. Be sure that the synthetic your are using is compatible with conventionals. Check the label on the container. As of this date, Mobil's and Castrol's synthetics are compatible with conventional oils. I recommend staying with one brand to ensure that there are no compatibility problems. Blending oil in this way gives you a higher percentage of synthetic base, plus the advanced additives, for less cost than buying a blended oil.
If money is not an issue, then just go with full synthetics.
In a word: "don't." There is very little that any additional additive can do to engine oil and be compatible with all types. I have read a lot about a couple of them and it has been discussed frequently on the SDML. Also, the price of the more "advanced" additives makes buying pure synthetic oil cheaper than buying conventional oil plus additives.
The more popular Slick 50 is a cheap base oil that has little bits of solid Teflon added. The fact is that engine oil needs to get into very tight spaces to lubricate properly, and if heavy engine oils have a hard time, imagine how well solids can get in. Basically they do nothing and end up getting filtered out by the oil filter as the filter becomes dirtier. So for about $25, you get a quart of cheap oil and a bunch of particles that really belong on a non-stick frying pan. Dupont (the manufacturer of Teflon) does not recommend Teflon for use in internal combustion engines, so why are people putting it in there?
The latest additive is Prolong. It appears that Prolong is some sort of chlorine-based chemical with buffers to suppress chlorine's corrosive tendencies. Details about this stuff are still sketchy, but it won't be going into my engine until I see real racers using it. If you are a Prolong believer and don't mind spending $30 on it, it doesn't seem to cause problems as far as I know.
There are various others, such as Duralube and STP. There has never been any conclusive evidence that any additives have any effect on engine wear at all. Some may even cause damage. Don't listen to the vendor's own tests or those stupid infomercials, because they are simply not true. The "running the engine without oil" test is nonsense. All modern engines require some oil pressure to keep the lash adjusters pumped up. It's simple physics. There is no way that an engine can run quieter with no oil after any kind of treatment (ala Prolong's infomercial). Even if the treatment has some magical properties, the lash adjusters require pressure to keep the valvetrain quiet! The million mile test is also nonsense. These tests leave the engine running constantly. Almost all engine wear occurs when the engine is started, not while it is running at operating temperature.
All About Oil by Mark Lawrence. A very in-depth look at oil.
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