CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Oil Life Calculation

I saw this post on the Reddit labeled version of Cartalk. The post;

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 * (Engine oil capacity in qts.) * average mpg over the period

So for my Mustang that is 50 * 8 quarts * 19 mpg = 7600 miles
My truck would be 50 * 6qts * 13mpg = 3900 miles

This formula accounts for the highway driver or short trip driver. Highway mpgs are higher so oil life would be longer, the short tripper would have much lower mpgs so the oil life would be shorter. Supposedly the formula is based on acid buildup in the oil so it doesn’t matter if this is synthetic, a blend or conventional oil.

The oil minder on the Chevy seems to trigger much later if driven lots of highway miles but only a little later for short trips. The Mustang is an anomaly as it gets the oil changed early because it gets tracked.

1 Like

Hellcat 50x7 qts = 350x 7mpg = 2450 miles.
Rear tires = 2500 mi.

So my Focus would be 50 * 4.5 * 32= 7,200 (although I’ll stick with 5k)
my Odyssey would be 50 * 5 * 21= 5,250, which is about right with the OLM

That’s an interesting formula

That’s a very interesting formula. Actually makes a lot of sense. …I like it a lot.

Only problem I have with it…most people can’t do simple math. :roll_eyes:

4 Likes

Agree! Most people can’t understand “5000 miles OR 12 months, whichever come first” either. :stuck_out_tongue_winking_eye:

4 Likes

It won’t work for hybrids, but otherwise, it can be a good rule of thumb.

Plug-in Hybrid…sure. But why not regular hybrid? Change interval will be a farther out, but the engine isn’t working as hard as regular ICE engine…so I think it could still be valid

3 Likes

IMO, the constant should be different to account for the electric assist. For early non PHEV hybrids it was probably still a good estimate, but as electric assist intervals expand, the constant 50 becomes less viable. In any case, I won’t substitute the equation for the OLM. Since it takes operating conditions into account, I’ll go with that.

+1
Based on a lot of questions in this forum over the years, it seems that adults (who probably hold responsible jobs and who most likely have the right to vote), are frequently unable to understand that… extremely tricky :wink:… phrase, “whichever comes first”.

My oldest friend, from my undergraduate years, believed that because he only drives about 900 miles per year, he could wait about 7 or 8 years before changing his oil. Only after a LOT of prodding (and explaining the “whichever comes first” proviso repeatedly ) did he finally get the first oil change for his Scion, after ~4 years. IIRC, he has now had the oil changed twice in the 7 years that he has owned the car.

And, even though he is the cheapest person I have ever known, all of his oil changes for the first three years would have been free, if he had availed himself of them. Go figure…

1 Like

One way to look at it is that a quart of oil can protect an engine while it burns 50 gallons of gas.
A 5 qt sump is good for 250 gallons of gas.

That formula doesn’t work for turbo engines. My 2018 Mazda 6 calculation would suggest a 7,500 mile OCI, but the factory interval for the turbo is 5,000 miles. With a turbo operating at high temperature, I would not chance extending the OCI. My older Mazda 6 (normally aspirated) easily goes to 7,500 miles between oil changes and is currently at 175,000 miles without any oil burning.

Simply based on this formula my mom’s 2010 Prius would be good for about 11,000 miles between changes, The 6,000-7,000mi intervals she’s doing now are worth the $55 annual oil change and service with the independent shop. Been following these guy’s lead since 1993 without issues.

Who said the formula applies perfectly to every possible case?
It’s just an interesting way to look at things.
It’s clear who on this forum I would or would not want at a brainstorming session.

2 Likes

But if t he electric range is pushed out then the gas mileage will increase…and thus push the number higher. I still think it’ll be close. Plug-in hybrids are going to be different tough.

Good point. So maybe adjust formula - 25 * (Engine oil capacity in qts.) * average mpg over the period

I still like the Formula and the way it looks at oil change interval.

My turbo Audi has a 10k oil change interval. By the formula, it should be changed at 7500 miles. So it is on the other side of Mazda’s 5k interval.

I wouldn’t go 10k on a change interval on any car let alone a turbo. Better to err on the side of caution.

3 Likes

Besides engine type (turbo, hybrid etc.) some engines just run dirtier or cleaner than others, mainly due to ring seal and blowby, and would benefit from more or less attention than any one-size-fits-all protocol.
And that’s whether we’re talking a formula like above, operating hours, or mileage.

1 Like

I go with the car manufacturer’s recommended interval, and try to do it between 15% to 20% life remaining. They spent a lot of money figuring that out, and I’m willing to believe they want return customers.

Nothing against the calculations but I do it the easy way. Every 5000 miles. Not much math involved. The extra cost is deducted from the inheritance to be passed on.

1 Like

I have a 2020 Ford F-150 with the 5.0 V8 and I had its first oil change done at 9,140 miles and the oil life monitor was at 12%. The oil looked dirty but not black. Also the maintenance schedule is 10k between oil changes OR when the oil change light comes on. So I would take the oil life monitor with a grain of salt. :smirk:

It is a rule of thumb, after all.

The 5.0 Ford has an 8 quart oil pan in trucks or cars. Doesn’t take much of an increase in mpgs to hit 10k changes. 8 qts × 50 × 25 mpg = 10,000

That is a only a bit more than my Mustang can get for highway mpgs

1 Like