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

#### leoblack9

Good day to you all. I would just like to ask if this advice which I found on another forum would be a viable approach (since some of us don't have a blackstone labs or oil analysis company at reach).

In my opinion this could be a good starting point to determine an oil change interval for your own car (which takes to account your driving usage) but of course has a lot of left out variables (synth oil vs dino, turbo vs n/a engine or gas vs diesel). Thread could be found here.

Engineer here for a major automotive company. An older colleague passed along this oil life rule of thumb before he retired. It's too good not to share. He had reviewed over his career probably thousands of sets of oil analysis data, and this RoT is based on that.

Oil life in miles= 50 gallons of fuel consumed per quart of oil capacity, times MPG.
-Or-
"oil pan capacity times 200 is the volume of fuel you can burn before changing oil."
5L oil pan would go 1000L of fuel burn before changing oil.

The idea is to calculate the volume of fuel you can consume in the oil service, then convert that to distance using your fuel efficiency. So if you calculate 1000L of fuel burn between changes and you get on average 8L/100km, you'd change every 12,500 km.

This rule gets away from unsophisticated and obsolete blanket statements like "every 3000 miles" or "every 5000 miles" and focuses on the primary cause oil degrades-- fuel combustion byproducts. Yet it's simple enough to use across vehicles and applications. It accounts of cold starts and short trips vs warm engine and hwy miles. It accounts for engine wear and power loss to some degree.

If it helps you feel better, you can collect oil samples and have the lab analysis done. Or you can get good-enough-for-most-of-us optimization with some very simple math. And if your vehicle has an oil life monitor, it's doing nearly the same thing but with electronic logging of throttle position and engine temperature and such. This rule of thumb will get you about the same place as an oil life monitor and can be used to sanity check it.

Finally, the 200 scaling factor (oil pan volume to fuel burn volume) can be fudged up or down if you think it is warranted. A Factor of 180 would be 10% more conservative, for example.

Caveat: this is not for race cars or other vehicles that sustain very high oil temperatures and have abnormal oxidation rates.

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NO way I would blindly apply that to a new \$30+K vehicle with modern oil wetted components.

This forum started in 2003 because there were some less than ideal oils out there that sheared down faster and/or left varnish deposits. That was a LONG time ago. Even then oils had gotten far better than the oils in the 60s - 90s which created lots of Myths and legends that were long obsolete but people still followed them.

Nowadays, Oils with the latest API rating are very good regardless of brand. "Synthetic" is not longer an expensive upgrade vs conventional due to Walmart's low price differential. if you run wal-thetic (M1, Penzoil plat, castrol edge/mag, QSUD, Supertech, etc) in an engine that calls for that grade of oil, you can look for other maintenance items to worry about.

Change it at the recommended OCI (5K seems to be the min now) and if you want to stretch it. take a \$20 oil lab test and see how your oil is after that run. If its really good, go ahead and stretch and test again. After about 2-3 tests, you should have a number you can stick with. Adjust for short trips, excessive city driving, or extreme heat (desert driving/ mountain climbs in low gear)

Its not like its a large investment to get the oil tested to establish a baseline then run the engine that way to the rest of the car life or until something major changes with the car.

I'm an engineer too. I use numbers and math all the time and I see where the author was going but I also am "seasoned" enough to see the practicality in things.

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Seems overly conservative for a market such as the US where vehicles use some sort of synthetic or semi-syn.

Probably as good as anything else someone pulls out of thin air.

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.

Pretty interesting - assume this pre dated the OLM

Good day to you all. I would just like to ask if this advice which I found on another forum would be a viable approach to some of us here from a place where there's no blackstone labs or oil analysis company at reach. In my opinion it's a good starting point to determine an oil change interval for your own car (which takes to account your driving usage) but of course has a lot of left out variables (synth oil vs dino, turbo vs n/a engine or gas vs diesel). Thread could be found here.
These are somewhat similar to TooSlick's formula that takes MPG and fuel burned into account. My 530i's OLM is based on amount of fuel burned as well, although the value they've chosen is a bit too high for my liking. But I like the concept of using amount of fuel burned as an OLM indicator in general as it automatically takes into account various factors such as running on cold engine, short tripping, idling, etc.

Code:
``https://docs.google.com/spreadsheets/d/1VChJk6rO-Wocwor_7_mpnGEyiCCuEmz9FaPtbcEhIC8/edit#gid=4``

This forum started in 2003 because there were some less than ideal oils out there that sheared down faster and/or left varnish deposits. That was a LONG time ago. Even then oils had gotten far better ..lots of Myths and legends that were long obsolete

Change it at the recommended OCI and if you want to stretch it. take a \$20 oil lab test and see how your oil is after that run. If its really good, go ahead and stretch and test again.
For many new vehicles, proper oil choice and frequent oil changes are exceedingly important. There is a reason many modern engines trash a timing chain at less than 100,000 miles, on the OEM oil and change interval. From Ford's 3.5, 3.7, 5.4 and others, Chevy's 3.0 and 3.6, various Dodge engines, BMW's and yes, even some Toyota engines. Not to mention cam phaser problems, bearing problems, balancer chain problems and so on.

It remains a wise thing to understand what engine you operate, and to know whether it's subject to rapid wear issues using the OEM oil and change intervals. On many of the above mentioned engines, a timing change job is \$4500.

Pretty interesting - assume this pre dated the OLM
Maybe even further back to the days of carburetors when fuel dilution was more of an issue.

Maybe even further back to the days of carburetors when fuel dilution was more of an issue.
Yep, certainly smelled fuel when my old PowerWagon had a Carter AFB …
I have a plan for GDI dilution … CBU ? Not so sure …

CBU?...the stuff from marijuana?

CBU?...the stuff from marijuana?
Not until I retire … meaning carbon from direct injection fuel bypass of ports etc

to determine an oil change interval for your own car
Change it at the interval the owner's manual says or when the oil-life monitor/indicator in the vehicle says. Only a tiny % of street vehicles use oil analysis labs and are by no means necessary. Even some people who use labs still ignore (or don't "trust") the mileage intervals they recommend or say can be used.

The numbers I get from this calculation are plausible. I run full syn oil and an oversize filter on an engine that specs conventional at 5k/6 month intervals (no OLM). But the MPG number used in this calculation must be what you are seeing *only* during the current service interval, not a CAFE rating from the manufacturer or other sources, not even your own average long-term MPG.

Seems overly conservative for a market such as the US where vehicles use some sort of synthetic or semi-syn.
it’s overly conservative for 1985

These blanket type estimates don’t work for everyone. It depends on driving conditions, length of travel, speeds, and so forth.

Since the OP is from the Philippines he could use the example of traffic in Manila which is some of the worst in the world. Would he go 10,000 km between oil changes driving that every day in a turbo GDI engined vehicle?

Just for clarity I have been to Manila and I can attest to the severity of the traffic.

These blanket type estimates don’t work for everyone. It depends on driving conditions, length of travel, speeds, and so forth.

Since the OP is from the Philippines he could use the example of traffic in Manila which is some of the worst in the world. Would he go 10,000 km between oil changes driving that every day in a turbo GDI engined vehicle?

Just for clarity I have been to Manila and I can attest to the severity of the traffic.
But that's the whole beauty of factoring in amount of fuel burned to establish an OCI - it accounts for most of these driving conditions. For example, driving in heavy traffic = bad fuel economy, you are burning through fuel but not accumulating too many miles. High speed driving - same thing, as it results in worse fuel economy. On the other hand, leisurely hwy driving is accounted for as well because you're getting good fuel economy allowing you to rack up more miles during an OCI.

Is it perfect? No, but it's better than some blanket X miles OCI statement.

But that's the whole beauty of factoring in amount of fuel burned to establish an OCI - it accounts for most of these driving conditions. For example, driving in heavy traffic = bad fuel economy, you are burning through fuel but not accumulating too many miles. High speed driving - same thing, as it results in worse fuel economy. On the other hand, leisurely hwy driving is accounted for as well because you're getting good fuel economy allowing you to rack up more miles during an OCI.

Is it perfect? No, but it's better than some blanket X miles OCI statement.

Point taken but as an example my CX5 gets 32-33mpg on the highway. That could drop to 27 in city for example but the result would still be a long OCI in my view. Now that is a personal opinion on my part and not scientific in any way.

It is a good point though.