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synthetic oil and oil viscosityFor 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 viscosityOil 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.checking oil

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!

synthetic oilThe 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 ClubRV 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.

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