Nearly everyone is aware that crude oil was formed by the decomposition of plant and animal life from hundreds of millions of years ago. This material was compressed under vast sea-bottom deposits and the pressure and heat created hydrocarbons: that is, mineral, or crude, oil.
Lubricating oils are the product of the refining of crude oils. These oils are materials made up of many chemicals, including detergent and paraffin. Paraffin (wax) is an inherent component of crude oil and not all is refined out. Paraffin is the chief reason oil thickens up when it gets cold. Anyway, back to our engines...
Engines would quickly seize-up without lubrication and oils are designed to prevent bearing wear and metal-to-metal contact. This is accomplished by pumping the oil under pressure so that it forms a film between those parts. Manufacturers put special additives to oil to allow it to perform properly over varying engine conditions and time. Some additives slow thinning as engine temperatures rise, hence the "multigrade" designation on the container. Other additives help to limit the breakdown of oil under loads.
Internal combustion causes byproducts such as water and carbon. These byproducts are what create the "dirt" in oil, necessitating scheduled changes. These can create deposits and acids, so other additives are in place to do the following:
Antioxidants are there to minimize the formation of acids, varnish and sludge.
Detergents are added to keep engine parts clean by lifting deposits (mostly carbon) from surfaces.
Corrosion Inhibitors also work against the formation of acids that can attack bearing surfaces.
Rust Inhibitors keep water from mixing with the oil film and causing rust.
Foam Inhibitors prevent the formation of foam bubbles in the oil, due to detergents and the physical churning of the oil during engine operation.
Friction and Wear Reducers, Dispersants are there to help keep insoluble combustion byproducts in suspension and to minimize wear in bearings.
All mineral oils are graded by the Society of Automotive Engineers (SAE). These grading criteria are based upon the expected load and temperature ranges under which a given engine will be used. Viscosity grades are available in a wide range. These grades are most frequently seen in forms such as 5W-30, 10W-30, etc., in multi-vicosity blends. Straight viscosities are also available (20 W, 30W, etc.).
It is true that most engine wear takes place during startup, since virtually no measurable wear takes place during running. The faster an engine is revved during startup, the more the wear, so keeping a light foot on the accelerator pedal during startup — and warmup — is the secret to long engine life.
Oil is viscous and has capillary action. That is, it remains on the engine parts as a film indefinitely after running. Contrary to advertisements, your engine's parts don't scrape against each other (as bare metal) just because you haven't started it in a few weeks (granted, if it's 10-degrees outside and you haven't started that '59 Corvette in six months, there will be a little wear when you start it up. However, if you crank the starter for 30 seconds or so and then start the engine, little significant damage will result because you will have pumped oil into the system under minimal loads).
For decades, however, a huge industry has made billions of dollars by convincing the public that their engines are wearing out prematurely due to lack of "complete" lubrication. The aftermarket oil additive industry — there are many of them and they advertise everywhere — claims to show "proven" results, longer life, better mileage, etc. by using their products. None of this is true.
What these marketers are selling falls generally into two categories: either their product is a variety of soluble chemicals (chiefly, chlorinated paraffin) or solid particles suspended in a "carrier." The chlorinated paraffins work fine for your drill press (cutting oil is chlorinated paraffin) but don't do anything in your engine. Solid particle additives, chiefly PTFE or Teflon, is a suspension of ground-up particles in oil. It won't work, period.
Many of the aftermarket additives contain compounds already added by the oil refiner (No, more is not better! The refiner put in the right amount for the job and more would only upset the mixing proportions of the other additives!). Other additives contain "questionable" materials that could harm engines. Engine oils are complex mixtures and adding the wrong chemical compound to a properly formulated blend can cause failure of other critical additives.
Your engine was designed by competent engineers and a lubricating oil was specified for it. Properly maintained (and brought up to correct operating temperature at each use), it will easily last 150-200,000 miles on ordinary mineral oils. Under normal operation (startup to redline) the extremes of load, temperature and wear claimed by additive advertisers don't exist in your engine.
We can't stress the following enough:
No oil manufacturer recommends the use of off-the-shelf additives in their products, nor do any automobile manufacturers. Most additives do nothing (remember, those high loads and stresses don't take place in normal engine operation) and some actually can contaminate the oil. No manufacturer's warranty will cover failure if it is found that an additive was present in the engine oil.
Virtually all additive advertisements (and synthetic oil ads as well) claim better fuel mileage. This is anecdotal, unprovable and non-scientific (before you write back with your rebuttal, consider this: if any claim of better mileage could be proved to be repeatable, the government (NHTSA, in fact) would require the manufacturers to use the product. The manufacturers themselves would specify it because it is all they can do to reach those CAFE — Corporate Average Fuel Economy — standards). It just ain't true.
So you used an additive once and found the engine ran better and you got better fuel economy? Of course you did, because you were paying closer attention to its operation than you normally would! This is known as the "Hawthorne Effect" and it works because of that very fact. If you collected random records of the engine's performance you would find no change from before.
Today's motor oils are very high-quality products. However, synthetic oils are becoming very popular, especially used in high-performance engines. Synthetic oils are blends of synthesized hydrocarbon fluids (SHF) and esters, all derived from the refining process. The chief material of synthetic oil is polyalphaolefin, derived from ethylene. Synthetics can be called petrochemicals, and while they appear chemically similar to mineral oils they are "pure" chemicals. No waxes or other impurities are present in synthetic oils. Since they don't contain parafin, synthetics tend to flow better at lower temperatures and not thin out as much at higher temperatures. They are available in multi-viscosity grades from 0W-30 to 15W-50.
Synthetic oil has some advantages:
Relatively constant viscosity over a wide temperature range
Higher detergent properties. They are naturally detergent
Non-toxic when handled properly
No harmful impurities
Better lubricative properties
Less breakdown tendency under high loads
Synthetic oils cost about three times that of regular oils and aren't really necessary for most people. There are some situations wherein the extra expense of synthetic oils is warranted, however.
1. Racing: Engines are run full-time at high rev and there is tremendous heat. You need all the help you can get.
2. Extremely cold environments. Maybe your engine won't know the difference, but you'll save money on batteries.
3. Engines which don't use camshaft bearings (Lamborghini,Ferrari and others, including 30s and 40s cars).
4. Engines that won't regularly get up to proper operating temperatures. Improper warm-up is the biggest killer of engines. Cold sludge and varnish will build up and create lots of friction, hence wear and failure. Synthetic oils will not form these deposits.
5. Carbureted cars that run exceptionally rich (if the oil dipstick smells like gas, yours is one).
It can't be mixed with regular mineral oils. Not so. Mobil 1 and the others are perfectly compatible with mineral oils (but not with each other).
You get longer oil life with it. Not so. You have to change your oil according to manufacturers' instructions (it's the exhaust byproducts which get into the oil and turn it black, requiring change. Synthetic oil can't hold any more of these particles than mineral oil).
Synthetic oil will destroy seals and gaskets in older vehicles. Not so. If the seals are functional no harm will be done. If they are hardened and shrunken, you need to rebuild the engine.