# Oil life Rule of Thumb from another forum. Good advice?

Sure, but do they have a vested interest in engines that last several times the warranty period
You're suggesting they tell owners "change the oil every 10k miles" so that the engines will die of oil-related failures after 60k miles or soon after ? And if that were to happen, people will run out and buy another model from the company ?

Don't get me wrong, if someone wants to change synthetic oil every 3000 miles, more power to 'em ! This site, other sites, oil analysis results, and so on have shown that engines aren't dying from using manufacturer-suggested oil change intervals.

Vehicle: 2007 Volvo V70 2.435l non-turbo, auto.
Formula #1: 50 X 6.1 qt. X 27mpg = 8,235 mi.
Formula #2: 6.1 qt. X 200 = 1,220 qt. / 4 = 305 gal. X 27 = 8,235 mi.

A lesson in cost/use calculations says that if you employ different formulae and get the same or near result, you're likely accurate.
A dead match would be eerie but the use of 50 in one formula and 200 in the other correspond to gallons and quarts respectively.
This is more a parameterized math exercise.
You are doing the same math in both formulas so it will be the same result. 200/4=50. 4 quarts to a gallon.

Yeah traffic here is absolutely terrible. The rule of thumb that everyone follows here is 10,000km or 6300mi maximum if using synthetic.

Using the formula my diesel Toyota (Fortuner which is a common rail 2.5L turbocharged unit) gets 16.45mpg in absolutely horrible Manila traffic; has a 7.5L sump (oil pan + oversized Fram ultra xg3600) equals 6172mi or 9875km. Pretty near the hard 10,000km rule.

Food here is great if you have a thing for rice meals, bulalo and sisig ftw.
I spent a few years there myself and it’s not only the traffic you have to deal with but also the year round burning, especially in dry season. A lot of particulate matter being sucked into the intake.

Do miss the food!

I'd suggest an adjustment for boosted engines...

like perhaps use 0.80 times the 50 times sump capacity times MPG

or 40 x Sump in qts. x MPG.

But I just made up the 0.80, but it should probably be something a little less than 1 since turbos and superchargers are harder on oil than a NA engine.

Static thinking will get you trouble in a dynamic world.

Static thinking will get you trouble in a dynamic world.
It's based on MPG/fuel burned during the course of an OCI, so it is dynamic.

You could just keep track of total amount of fuel poured into the tank, alas, most people will find it inconvenient, unless the car's computer tracks it for you like mine does.

So, to summarize, the formula is:
OCI (miles) = Oil sump capacity (quarts) x 50 x MPG
One can simplify it another step: 50 gallons of fuel per qt of oil changed.
Just keep a log of how much fuel goes in the tank and disregard miles.
My Tucson has a 4.5qt sump, so by this RoT I change oil every 225 gal of fuel.
EPA mpg is 23-30, so OCI mileage would come out to 5175-6750.
If I only drive to one of several stores within 2 miles (thanks COVID!) I get 17mpg; OCI would be 3825.
By using the RoT I wouldn't have to think about driving conditions, just keep track of fuel used.

One can simplify it another step: 50 gallons of fuel per qt of oil changed.
Just keep a log of how much fuel goes in the tank and disregard miles.
Yup. I mentioned the same just above.

Yup. I mentioned the same just above.
I was typing when you posted yours.
Saw yours after I posted.
Minds in sync.

You're suggesting they tell owners "change the oil every 10k miles" so that the engines will die of oil-related failures after 60k miles or soon after ? And if that were to happen, people will run out and buy another model from the company ?

Don't get me wrong, if someone wants to change synthetic oil every 3000 miles, more power to 'em ! This site, other sites, oil analysis results, and so on have shown that engines aren't dying from using manufacturer-suggested oil change intervals.
No, that's not what I'm saying, exactly. All engineering decisions involve trade-offs, some of which might have nothing to do with engineering. Hypothetically, if the pointy heads tell the bean counters, "If we lowered the oil change interval from 10k miles to 7.5k miles, the engines will last 10% longer on average," and the bean counters say, "No thanks. If we make it 10K, we can offer more free oil changes and sell more cars." Does the manufacturer really care if the car only lasts 15 years instead of 20?

It's the same reason that they tell the consumer the transmission and differentials are "Fill for life" and don't require service. Anyone with half a brain knows that isn't going to give the best service for the consumer.

Edit: Sorry if that sounded like I was making statements about your brain, Hall. not my intention at all.

I wonder how close this formula is to the Honda maintenance minder? I usually change oil at 5k and the minder is always at 15-25% at the change. I still haven't figured out the effects of idling, driving fast, etc but assume these factors are included.

I still haven't figured out the effects of idling, driving fast, etc but assume these factors are included.
I suspect that in the grand scheme of things, those aren't as much of a negative factor as people tend to think.

Why, you think Ford didn't test it extensively and are confident that it's completely safe for the engine ? They have no interest in destroying engines that in many cases will need replaced under warranty, i.e. on their dime.
Most vehicle manufacturers, especially in the US try to extend intervals to sell it as being cheaper to own. Technically even a Hyundai will last 60k on the factory oil change before it self destructs, most modern engines will go 100k even with major neglect. If you want your engine to last as long as physically possible, stretching oil changes much farther than 5k (or 7.5k in ideal 90% highway driving) probably isn't a good idea.

stretching oil changes much farther than 5k (or 7.5k in ideal 90% highway driving) probably isn't a good idea.
blanket numbers at it again

If you want your engine to last as long as physically possible, stretching oil changes much farther than 5k (or 7.5k in ideal 90% highway driving) probably isn't a good idea.
Generally speaking, oil manufacturers, automakers, oil analysis labs, etc disagree.

Don't get me wrong, if someone wants to change their oil every 100 miles, go for it. If they change it every 3000 miles, go for it.

I have been following this thread, and think the formula is good, but am wondering if time has a factor in all this. As in my cars that are only driven in summer long trips, but only 1000 or so miles per year. And my daily driver that gets lots of short trips and only a monthly long trip year around but still takes a year and 1/2 to 2 years to get to the miles that the formula figures out too.

... You could just keep track of total amount of fuel poured into the tank, alas, most people will find it inconvenient, unless the car's computer tracks it for you like mine does.
Or you log fuel purchases. You have to add them all up anyway to know your true average mpg over any oil change interval. Simply going by fuel volume consumed is simpler overall than circuitous math using mpg to reach the same result.

I'm changing oil about every 150 gallons, in effect.

Especially with GDI/TGDI. Lots of Ford's sludged up at high mileage following the recommended interval. For the first 100k of my GDI Hyundai's life I did 5k intervals and still ended up with some minor deposits (most of which have been removed with shorter intervals, seafoam, and a few ester based oil changes).

Any small displacement, high output GDI or TGDI engine I own in the future will be getting 3-4k intervals, especially if it's got a small sump.
Now that I have found an acceptable oil for my TDI engine I will change it every 5000 to 6000 mile even though OCI says about 7500 on a Honda.
Even with top quality oil I use I get itchy to want to change it out when it hits 4000mi. I went to top tier oils years back due to being disabled and since we put so few miles a year it makes sense to use the best and try to stretch the intervals. I just pay my nephew to stop by about two to three times a year and pay him rather than some shop. It is extremely hard for me to trust ANY mechanic shop these days with the poor quality of work I hear about and the stories I am told by Service Managers I know who can barley bribe young men today to take on those great paying auto tech jobs. Its sad the majority of drivers today cant tell a torque wrench from a tooth brush. Most of them would probably have trouble changing a flat tire these days.

Using this method with my average MPG over the past year my Versa OCI would be about 8600 miles. 3.1875 qts. x 50 gal./fuel x 53.947 MPG

I found this OCI calculator and it says it should be about every 7900 miles. https://atlib.info/calc/oil-change-by-fuel-consumption

Manufacturer interval is 5K miles which using the method listed in this thread and using EPA combined MPG of 34 MPG the interval would be about 5400 miles.

The oil in the Versa looks great doing a blot test about every 1K-1.5K miles so I've been doing 7500 mile intervals for the last couple OC's. My blot test on the 7500 mile oil looks as good as "oil of properly functioning ICE with short run period" shown here. https://atomium.eu/home/atomium-articles/condition-based-lubricating-oil-change/ On my last 7500 mile OCI I'd used about 130 gallons of fuel.

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Why, you think Ford didn't test it extensively and are confident that it's completely safe for the engine ? They have no interest in destroying engines that in many cases will need replaced under warranty, i.e. on their dime.
LOL. They are in business to sell new cars after they squeak the purchased ones by the warranty period.

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