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For years, the standard in changing engine oil has been every 3 months or 3,000 miles, no exceptions. Then along came synthetic oil which advertised going 5,000-7,000 miles before changing and the debate began.
Motor oils main function is to lubricate moving components in the engine as the heat that is generated would cause the components to expand and lock up. The motor oil creates a separating layer to protect these components which reduces heat and friction.
To start, let’s take a look at the composition of both conventional oil and synthetic oil.
Conventional oil is made up of 75-90% base oil and 10-25% additives such as detergents, rust inhibitors, corrosion inhibitors, and other components.
Synthetic oil starts as base oil refined from crude oil just like conventional oil, but then is distilled down to the molecular level to eliminate impurities and then is customized with additives by the oil manufacturer.
Oil is graded by viscosity which is the ability to flow in different temperatures. The Society Of Automotive Engineers (SAE) has developed a test to measure and grade the flow of oil through a pipe and assigns a number to coincide with that flow. The higher the number, the slower the flow rate. Conventional oil with a single number such as SAE 30 has high viscosity, but will be difficult to start in cold weather. These ratings are sometimes referred to as “weight”. Using a lower weight oil such as SAE 5 would be easy to start in cold weather, but does not provide enough protection when the engine heats up to 210 degrees operating temperature.
Therefore oil manufacturers have developed multigrade viscosity oils that will be easier to start in cold weather and provide better protection during operating temperatures. For example, SAE 5W-30 has a base weight or viscosity of 5 for easier start up in cold which the “W” stands for winter, and a higher viscosity of 30. Many people think this is simply mixing two different viscosity or weight oils together, however it actually starts with the lower viscosity base oil and additives are used to increase the viscosity. Components such as Pour Point Depressants (PPD) and Viscosity Index Improvers are able to increase the viscosity at higher temperatures. Basically the base oil starts with smaller molecules or polymiers that flow by each other easily when cold, then expand with the heat and lump together creating a higher viscosity.
When To Change Oil
Today, most engine manufacturers rely on the onboard computer to regulate the usage, temperatures, and other factors to determine the proper oil change interval. The “Change Oil Soon” light indicates when it’s time to change the oil. Diesel engines have much longer oil change intervals than gasoline and some diesel models still recommend an actual mileage/time oil change interval. Check your chassis owner’s manual for recommendations. In my truck, I stick with the “Change Oil Soon” light. In a motorhome, since you typically don’t put as many miles on, I suggest changing a little more often. This might seem a little counterintuitive but oil and other fluids break down over time. So, a vehicle that spends a lot of time in the driveway will probably need an oil change more often than a vehicle that is driven often.
Conventional vs Synthetic
For years it was a heated debate on the ability to use synthetic oil or not. Most engine manufacturers fought the new oil stating it would void the warranty and could not be “mixed” with conventional oil and would not protect the engine! As stated previously, synthetic oil actually starts as base oil and is distilled to a much cleaner oil so all the hype is just that…hype and not true. You can switch to synthetic and even back to conventional if necessary without damage to an engine and you can even put a quart of conventional in an engine that has synthetic if you are in a situation where synthetic can not be found and you are low on oil. However, it’s a good idea to carry an extra quart of what you do have in the engine as you travel!
The advantage of synthetic oil is it provides better protection longer. Even though both conventional oil and synthetic have similar additives such as anti-wear, anti-foam, detergents, rust inhibitors, and corrosion inhibitors, the synthetic provides superior protection at the end of the oil change interval. Typically synthetic manufacturers don’t claim the oil extends oil change intervals, the real advantage is synthetic maintains a superior flow from new to used several months/miles down the road.
When I was young driving cement truck for a local company, an oil representative came to our location trying to get the management to switch to synthetic oil and conducted a very impressive demonstration. He filled three Styrofoam cups with various conventional oils and one with his synthetic. He then poured all the oil out of the cups and showed us there was still a small amount of oil on the side wall of the cups. He then set them on a shelf asking us to inspect the cups the next morning. When we got to work the next day the three conventional oils had a small amount of oil standing in the bottom of the cup and the sides where bone dry. The synthetic oil cup still had a slick film on the side of the cup. His statement was; with conventional oil, as your vehicle sits overnight, the oil coating the components in the upper portion of the engine will drain down to the oil pan and provide little coating and protection at start up the next morning. It takes several minutes for the oil to be pumped and circulated to the components causing friction and wear that will eventually cause damage. The synthetic still had a thin layer the next morning and provided protection during start up. Not necessarily University Research Data, but it was interesting. More interesting…we switched to synthetic that summer and they reduced engine issues almost immediately.
About the author:
Dave Solberg: Managing Editor, RV Repair Club
For the last 25 years, Dave has conducted RV maintenance and safety seminars, developed dealer and owner training programs, written RV safety and handyman articles, authored an RV handbook reference guide and logged over 100,000 miles on the road in an RV.
RV Repair Club is your go-to online resource for enthusiasts who want quality RV maintenance, repair, and upgrade information – a community where passionate RVers can come together to gather knowledge and share their experiences.
Learn more about RV Repair Club.
John H. ~ Great to work with. The people helping me were the best and kept calling me to make sure me and my wife were in a safe place and if the tow truck was there yet. It does not get any better than Coach-Net!
Where did you come up with this, “For years, the standard in changing engine oil has been every 3 months or 3,000 miles, no exceptions.” ? Source of the scientific and engineering studies that show this is true?
This is simply not true. We that were alive back when this happened ALL knew that it was the oil companies doing. And they paid the car companies to put it into their service and users guides. That suggestion wasn’t there previously, and it was 5,000 miles suggested, not 3,000. Did oil change? No. Did the engines change? No. One year is was a 5,000 miles (or after hard driving or dusty conditions) suggestion, the next a firm 3,000.
You are perpetuating a myth that doesn’t nothing for 90% of consumers. All it does is line the pockets of the oil companies and retailers. And it’s total bunk.
If you tell that lie often enough it becomes true too often.
You couldn’t be more wrong. I remember when the API Service Catagory was SA, today it’s SJ. An engine today would be toast if you used SA although it’s now obsolete.
John W Weston said:
jimhitek what are you talking about!? I’ve been driving cars, working on engines, racing cars for over 60 years. I have rebuilt tens and maybe hundreds of stock and racing, American, European, and Oriental engines. You claim oils and engines have not changed over the years. That’s truly bunk! Both have changed dramatically. 3000 mile oil changes were the standard for many years. My daily driver car’s book recommends 10,000 mile changes for “heavy” use and 20,000 for light duty use. No time constraints noted. I average a mere 3500 miles annually on my “America’s sports car” used only for pleasure and parades. I change oil annually on both cars. My 15year old Chevy Suburban is only used for towing a 12,000 pound RV trailer and travels 6000 to 8000 miles each year. Again an annual oil change only. All synthetic oil in all 3 vehicles. The Suburban ‘s 8.1 litre engine was inspected internally by a complete tear down this spring. Every bearing and every wear point was right on factory specs. No noticeable wear! Every piece of the original engine went right back together. No new parts were installed except for gaskets. Tell me that my ‘55 Pontiac would have been the same? Not on your life! The engine rebuild at 55,000 miles showed worn out bearings, egg shaped cylinder walls and a completely worn out cam shaft and that car had fresh oil and filters every 3000 miles or less during its life. Have oils changed over the years? YES! Have engines changed over the years? YES! Technology, my friend, is real.
I like how you disagree with one point I made about the oil change recommendations of years ago, and than agree with my point that oil does NOT need to be changed every 3K miles (my point was that that one time it went from 5K miles to 4K miles over night was driven not by science but by greed) and you yourself don’t change but once a year. Sure a specialty car used to race with should have the oil changed more often…probably…but even then, you have some scientific evidence that it should be? Truly, you are a great friend to me agreeing and all. Thanks!
Tom Saroch said:
Totally agree with John Weston. The engines & oils today are as different as comparing a model t Ford to a mag Mercedes. Remember the good old days when an engine was junk after 50K miles. Today 200K with regular maintain the engine is broke in , not broke down .
Please, the reason engines are better today is the better engineering and tolerances provided by CAD and computer controlled machines, not by using more oil. That would be stupid to think so.
JR Phipps said:
To add my two cents…Jim is right, the original recommendation was changed without a change in oil…maybe it was to prepare us for the new oils, I don’t know. But it IS interesting that when your car is new, still on warranty and the oil changes are free, the recommendation from the manufacturer is 5000 to 7000 or when the sensors tell you it is time. BUT when you get it off warranty (and the free oil changes) the dealers tell you “every 3,000 miles is best.” The only answer for that is greed(?).
Waste and pollution. Can you imagine how environmentally friendly cutting the number of oil changes by 50% would be? Going from 3,000 miles to 6,000 makes a BIG difference, Sure, a lot of used motor oil is now recycled, but a lot still goes on or in the ground and waterways. My diesel gets changed every 15K to 20K. It uses conventional oil (Shell Rotella). At 816,000 miles it still has no blow-by or bearing noises. I used to send samples to Blackstone Labs at 15K and the answer was always the same – good for another 15K or more. Both engines and oils have evolved – as has filtration. my 50’s Chevy’s and Studebaker’s had no oil filter or just a single bypass unit. Very different today. BTW, how does oil sitting in an occasionally used engine go bad? That is just another myth for oil companies to make $$$. Occasionally starting an infrequently used engine and letting it get up to operating temp is all that is needed to preserve the quality of oil until mileage change interval is met. Do ALL Synthetic oils come from dino stock?
Motor oil has to come from suitable chemical compounds, usually crude oil to start, through it could be chemically built from other available molecular structures if cost is ignored. True synthetic oil was and is chemically synthesized primarily from ethylene and other chemicals to create a pure base oil of uniform molecular size. Hydrocracked oil is allowed to be labeled as “synthetic” oil because it is similar to true synthetic oil, based on molecular uniformity. Oil additives aside, molecular uniformity has the single greatest impact on “viscosity.” And viscosity is the single most important factor of motor oil. Regular motor oil is made up of more random molecular chains. And as the long chains become broken, the oil loses viscosity (thins out). Smaller and more uniform molecular chains found in synthetic or hydrocracked oil are less impacted by viscosity loss. So, provided that the additive package does not wear out, synthetic motor oil does not need to be changed as often as conventional oil. In a nutshell, yes, synthetic labeled oils come from crude oil, the differences lie in how they are chemically manufactured. Thank you for mentioning oil testing. One test is worth a thousand opinions when it comes to analyzing oil life and the need to change it, to include the mechanical health of an engine.
Google “how often should you change your oil” and you will see 3000 miles WAS the standard. In the early to mid 60’s, I worked at Service Stations while still in High School. 2500 to 3000 miles or every 3 months was the standard and straight weights, SA 30 was the norm and we used 50W at the local drag strip. If the engines were gummed up due to poor or no maintenance we used non-detergent to prevent the oil galleries from clogging up and rerefinded oil because it was cheap. By the way, the “standard” was not necessarily the manufacturer’s recommendation.
Regardless, calling the author a liar is totally uncalled for and the subject is not worth getting your panties in a wad.
Terry Hennessy said:
I sold auto parts and Castrol 30w, 10w30 and 20w50GTX Oil from 73-80, and the spec was SF, many manuals called for SD/ SE, but found this on the chronology/ evolution timeline that shows we are now up to spec “SP” (See the GulfWestern PDF): https://www.google.com/search?newwindow=1&ei=vQxGW8X6F8vEjwSFsqP4AQ&q=sf+oil+spec+chronology&oq=sf+oil+spec+chronology&gs_l=psy-ab.3…93934.99579.0.100318.104.22.168.22.214.171.124.2363.0j15.15.0….0…1c.1.64.psy-ab..0.14.2080…0j0i67k1j0i22i30k1j33i22i29i30k1j33i160k1j33i21k1.0.iDV_hbBCXFA
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