Please note that this article was written by me, as an introduction to the Automotive Oils World.
First of all, you must Understand that the
Basically Does all these Things:
► First: Engine Oil stops all the Metal surfaces in your Engine from Grinding together and Tearing themselves apart from Friction, by Lubricating its internals.
► Second: Engine Oil transferes the Heat away from the Combustion cycle.
► Third: Engine Oil must also be able to Hold in Suspension all the Nasty by-products of combustion, such as Silica (silicon oxide) and Acids, also External contamination, such as Dust.
► Fourth: Engine Oil minimises the Exposure to Oxygen and thus Oxidation at Higher temperatures.
It does all of these things under Tremendous Heat and Pressure...
Part one: the Proper Viscosity & Thickness
(SAE Grade) Recommended for your engine.
Remember: The viscosity of a fluid describes its resistance to flow. Heavyweighted fluids such as honey have high viscosity and flow very slowly, while Lightweighted fluids such as water have a low viscosity and flow much faster under the same conditions & temperatures. But in the Motor oil World, Viscosity is somewhat confused with Thickness, and those are two different things with different meanings.
☼ Viscosity depends strongly on temperature and is a major factor in the Flow of the oils. In example, the specification “10W~30” (“W” means “winter”) in a motor oil means the viscosity of the oil at 0º C (32º F) is no more than 3.1 poise while the viscosity at 100º C (212º F) is no less than 0.1 poise.
☼ Thickness of the oil is about the oil Barrier it could lay, between moving metallic parts to prevent Shearing; not about the Flow. It is the "cushion" that bears the friction between metal parts, the thicker number means thicker "cushion".
Obviously, hot oil will flow better than cold oil.
► Multigrade oils have the ability to get Thicker Molecules with heat to compensate their fluid's weight reduction due to heat exposure; in example: A 10W~30 Oil will behave as a grade "10" oil does at Ambient Temperatures, and will become thicker and behave as a grade "30" oil does on heat, when the engine reaches its normal operating temps.
So those oils with a "W" on their SAE Grade, are "Multigrade" Oils, (W for Winter times) their first number on their SAE grade is the "cool temperatures" number: Thinner oil helps to the engine's oil pump to move the oil faster during cold starts when engine is Cool and thus means less shearing by friction during cold starts. But the Oil's molecules will become thicker with heat, so the same oil will behave as the Second Number, intended for "hot temperatures" on their SAE Grade, when the engine Reaches its Normal operating Temps, in order to have enough protective oil film "Cushion" between moving metal parts and prevent shearing.
Want to learn How the Oil "Thickness" protective layer prevents Shearing on your Engine? ... Download the .pdf file document at the Bottom of this Page, you can see tests' results with Photos on it.
That oil barrier thickness increases with Heat due to the Polymers that where added to the Oil, those absorb temperatures; also Thinner Oils will increase the Oil Pressure on the engine and that is really needed on Cold Starts, to help the Oil to Reach the Farther engine's places, Faster.
How do Multigrade oils get Thicker with heat?
The answer was given by Mobil 1:
Multi–viscosity oils contains polymers called viscosity modifiers and these polymers act to thicken an oil as it heats up in order to provide the high temperature viscosity as in the case of a 5W-30, the 30 grade high temperature viscosity. When these molecules cool they coil up and reduce their thickening properties to give you the low temperature starting and pumping viscosity of 5W oil. That is how the oil can act as a 5W- (low temperature) 30 (high temperature) multi-viscosity oil.
► Monograde Oils (Single SAE Number) will not vary at all and will behave the same at all the temperatures' range; unless it becomes Very hot, when its Thickness 'Could be' Reduced, it is known as "Oil Break Down"... (more noticeable on Cheap brands with poor Quality) ...That is the inverse situation and could be Harmful for a Very Hot Operating or for an Overheating Engine: That oil under that conditions could Lost its main Lubricating Properties.
I Really advice against the use of Monograde oils on Cars, because they Won't flow properly on Cold Starts, remember: Those parts that worns faster inside the Engine are those parts that doesn't get Oil fast enough with Proper Pressure; so a Multigrade Oil will Reduce both the Time working without Oil on Cold Starts and the low Pressure.
Monograde oils could still be good for Power Plants, 18 wheels' trucks, etc... But Not ideal for a Car's engine. Keep the Proper SAE Grade in Mind when you Buy your Next Oil.
Part Two: The Motor Oil's Quality.
The American Petroleum institute API Has Two ways to classify Motor Oils: the Gasoline Motor Oils are clasified under the "S" Letter (From "Spark" combustion engine) While Diesel engine oil is clasified under the "C" Letter (From "Compresor" Combustion engine)
The API has a program to certify that the Motor oils, meets the strict Performance and quality standards put in place by the OEM. The Service Rating is shown in the API “Service Seal” on the product label, that may look like one of these examples:
So, The letter that follows the "S" or the "C" will let you know if the Additives Package (Detergents, Dispersants, Stabilizers, etc... ) is Better or Worse for your application. Any Letter from "A" to "Z" could be next to the "S" or "C" Letter, the more Newer Classification will place a higher Letter there, as Follows:
GASOLINE motor oils must read as Follows:
- SA = Early -older- motor Oils, Very Basic and without any Additive.
- SN = Very Recent Classification, includes a Complete Additive Package for Gasoline Engines under the APi Norm.
DIESEL motor oils must read as Follows:
- CF = -or any other Letter instead the "F" like "H", etc.- (Could have a Number 2 or 4 Next to it) The CF or CF-4 are for Normal (four Stroke) Diesel engines, while CF-2 oils are for Two Stroke engines.
So, if a DIESEL Motor Oil has, Beside its CF's or CG's or CH's Diesel oil APi classification, any "S" Classification, you could pour it also on a Gasoline Engine, if it Doesn't have any "S" (such like SL or SH or SM) Classification, then Avoid pouring it onto ANY Gasoline engine, those are "Diesel-Only" motor oils.
I've used Diesel oil on Gasoline engines for Years, with Great Results; if you want to do so, you should check if that Diesel Oil carry the Additives Package for gasoline engines too, if not, AVOID use it.
I Kindly Suggest you to Check ANY Motor oil to see if it has the Round Seal from the API ... as the above posted Examples. And the Proper SAE Grade inside that Seal's Circle.
If you Pour a "Diesel Only" Oil in a Gasoline Engine, it could be somehow Harmful for the Engine in certain way... While the Diesel Oils with gasoline Additives Pack included, (usually known as "Fleet" Oils) are Very Good and Outstanding Oils for Gasoline Engines, they also add "Zinc" to the Additive pack, that normally a "Gasoline-Only" motor oils lack to have.
On the Other Hand: if you Pour just "Gasoline" Non-diesel motor oil on a Diesel engine, the Oil will be Blackened almost inmediately and will fail for Proper Lubrication and Protection of the Metallic Surfaces, Beside other malfunctions; because the Gasoline oil will absorb the Harmful particles that Diesel Combustion filter to the oil and it couldn't manage that because Gasoline Motor Oils aren't intended for such contamination.
The newest Service category Rating for Gasoline engines in 2012 model year cars and light trucks is “SN.”
The API SN rating is equivalent to the new GF-5 oil rating by the International Lubricant Standardization and Approval Committee "ILSAC" the new SN and GF-5 rated motor oils are backward compatible and can be used in any older engines.
For Diesel engines, The current category is “CJ-4” (introduced in 2007 for newer diesels that have exhaust gas recirculation). The previous CI-4 (2002), CH-4 (1998) CG-4 (1995) and CF-4 (1990) categories all can be used in older four-stroke diesel engines. CF-2 (1994) is the API classification for two-stroke Diesels.
Part Three: Synthetic Motor Oils.
Synthetic Motor Oil... It's a confusing topic, and there's a lot of Rhetoric, largely Because some Manufacturers and Peddlers of Synthetics have made a lot of inaccurate and self-serving claims over the years. Some, actually a lot, of this Rhetoric is pretty Strident and Opinionated.
The problem that Most People encounter when switching a High Mileage car to Synthetic Oils, is due to the Detergents in the synthetic oil.
The conventional oil "Gunk" accumulates Around the Gaskets and other Places, often times preventing leaks... The Synthetic oil begins to Remove this "Gunk" and things like Oil Pan Gaskets, Main Seals, and Valve Cover Gaskets, start to Leak oil.
Now there are "High Mileage" Syntetic motor oils, so the Leakage problem could be, somehow, controlled ... ... But another thing to Consider is, Based on your Car's year model and type of cam / lifter configuration... a Roller Cam system works fine with Synthetic oil; However, with the removal of Zinc from Engine Oils over the last few years, many Flat Tappet Cams have been unhappy with this change.
Most Synthetic oils Doesn't contain Enough Zinc and are Not additive friendly ... ... (that is the reason why I preffer Diesel + Gasoline Certified Motor Oils: They do have Zinc, as I stated above).
Briefly, there are Two Types of "synthetic" Oils on the market.
► Group IV oils consist of Molecules that are synthesized from simpler chemical compounds. This lets the Chemical Engineers to "tune" the characteristics of a lubricant to exact specifications.
These oils are "Fully Syntetic" and flows more Freely at extreme Low temperatures and don't Break Down at very High temperatures; also they generally can be specified one or two grades Lighter than a mineral oil, which consumes less energy and saves Fuel. (Energy Conserving Oils)
► Group III Oils are made from Reprocessed petroleum products normally left over after making Crude oil into Gasoline, Diesel fuel, Heating oil and other products... so they're "Half Syntetic" oils, or Syntetic Blends.
They're more modestly priced and have many of the desirable characteristics of the higher-priced Spread. In much of the World outside the USA, Group III-based lubricants are not allowed to be marketed as "synthetic."
Don't assume that if is a Synthetic Oil, it is so Good... (Read: very Expensive) ...that you don't need to Change it as often. The base lubricant may be way Better, but the Additive package... (which can be as Much as 25 percent of the Volume of the Product in a Bottle) ...can still Become Exhausted, especially if you drive on Dirty / Muddy \ Dusty enviroments, due to Oil Contamination.
Also: Unburned Fuel, Partially Burned Hydrocarbons, Atmospheric Dirt, Metal Wear Particles and Blowby Carbon Particles will Build Up just as fast in a Synthetic-Lubricated engine as in one with petroleum-based oil ... ...
The Only Way to Remove all that dirt & contamination is to Drain and Replace the oil.
I've always recommended 3000-Mile oil Change intervals in offroading driving conditions, despite if you run Synthetic oil or not, and much more extended mileage for City Drivers.
So, with that facts in Mind, To switch to Syntetic motor oils on older engines intended for the Average Driving needs, does Not make any sense; Those syntetic oils are Better for Newer engines... (with Closer tolerances and better PCV systems which keep their oils Cleaner) ...or when the Car is build for Race or Special purposses; but otherwise if you offroad frequently, or drive in dirty enviromants, you'll be Wasting a lot of Money unnecesary, because you'll need to Drain the Oil around each 3000 Miles to keep the Oil with proper Lubrication and the Engine internals safely clean.
Part Four: Motor Oil Flush.
I Suggest to use a Motor Oil Flush Cleaner, at least every two years if you use your car for Offroading purposses or drive in very dirty enviroments. I use this from Motul on my BumbleBeast's Weberized EA82 engine, because I Drive it very Hard, 40% off-Roading (on weekends) and 60% in City Streets. (is my Daily Driver).
It is Amazing how many Mud, sludge and Dirt those Cleaners could Remove from the Engine, especially if you has never used one and the engine has high mileage. You only need to pour a Bottle to the Old oil Just before changing it, and let the engine idle for ten Minutes... (Follow the Directions on the Motor Flush Bottle, it might vary on Different Brands) ...then Flush the old Oil, change the old oil filter, pour fresh oil and Voilá!
But I Kindly Suggest you to Not do such Chemical Flush very frequently; if you do chemical cleansing too often, some seals could suffer a leak due to several reasons which includes: Attack from chemical agents and / or too much Gunk removed. If you Drain your engine's old Oil on a Regular Basis, there shall not be needed a Chemical Motor Flush, in theory ... but every two or three years isn't bad at all, and could aid to engine's longevity while keepin' the internals and tiny oil passages / hydraulic lifters more clean.
I Hope this Writeup will Help you to determine which motor oil is the best for your engine, because Motor oils Shall not be Choosen by Brand nor Marketing, but by the API Classification and SAE Grade considering the Specific Application where they're Needed and the temperatures where they shal work.
You must choose between the Oil Viscosity, Thickness and Additive Pack that best fit your Engine's age & design, your Kind of Driving and the Climate of the World's area where you Live; so the Smart choice is always to choose the better motor oil to Achieve the proper Balance between flow and usage, because a very thin motor oil will help you to save Gas (Energy Conserving) also will Help in Cold Starts in Cold Climates, but will worn faster the engine's internals on Hot Climates than a Thicker motor oil; but if you go too thick, the low flow pressure during cold starts on Cold Climates will worn things faster too.
That is the Reason Why car makers put some different Oil Viscosity Numbers to choose from, on their User's Manual; the Thinner oils are suitable for Winter times or Cold Climates, while the Thicker Oils are suitable for Summer Times or Hot Climates.
These are Scans of my 1983 Subaru Owner's Manual:
I Found an important Document which have a Professional Oil Test, done between some Different oil Brands and between Petroleum Based Oils and Synthetic Oils; they compare the Wear on a metallic part after the Test, using each Brand of Oil with same procedures for all; it has Pictures, so you can Easily "See" which oil has the Worse (weak) protective Film Layer and which oil has Better (Stronger) Protective Film Layer, between metallic moving parts, so you can better have an idea.
You can Download the Document, a .pdf File with less than 5 Mb, Here:
Edited by Loyale 2.7 Turbo, 17 July 2014 - 10:46 PM.
To add the Download Link ;)