The Oil Life Rule of Thumb

orange150

orange150

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Fairfax City, VA
Ripped from Reddit.

https://www.reddit.com/r/Cartalk/comments/k93n5f/the_oil_life_rule_of_thumb/

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.


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.

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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My little F150 averages 18mpg and holds 5 quarts:

5 * 50 = 250

250 * 18 = 4500 oil change interval
 
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This can't apply to diesels either. My oil change interval would be around the 11k mile mark. o_O

Leave it to an engineer to try to overcomplicate something because science, and then it not work in the field. :flipoff2:

Sticking w/ my every 5k on the 5k
 
This calculation is probably pretty accurate, i have a skiboat with a 350 merc in it. They say change it about the 50 to 70 hour mark. Since it is under so much of a load (burns 4-6 gallons an hour) the oil gets fuel contaminates way before it gets burnt. Which always shocks me when people fun a full synthetic and change it 50 hour mark.

In my 06 ram,the book recommended 15k mile oil changes or 7500 for severe duty. I always changed it about 10k with regular rotella. Since i did a mix of DD duties and towing. My 13 emissions intact i think the book says 7500 , but i typically do sooner since i dont put that many miles a year on it.
 
rcalexander105 said:
This can't apply to diesels either. My oil change interval would be around the 11k mile mark. o_O

Leave it to an engineer to try to overcomplicate something because science, and then it not work in the field. :flipoff2:

Sticking w/ my every 5k on the 5k
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You should start changing your tires like that with your mentality and waste even more money :flipoff2:


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NickMaul said:
You should start changing your tires like that with your mentality and waste even more money :flipoff2:


Sent from my iPhone using Tapatalk
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Good oil is clean oil

Oil is cheap (relatively speaking). Downstream effects of not keeping up w/ that shit on a 6.0 PSD is NOT cheap :flipoff2:
 
Anecdotal...but my grandfather who was a 40 year ford master mechanic, always said 12 months or 12,000 miles (sure there are arguments surrounding additives and tolerances). He retired in the 80's. As I got older and always heard 3k OCI's...it got me thinking, so I started to routinely get my oil analyzed at Blackstone. I started at 3k and eventually worked my up to 40k intervals, across a few different vehicles, personal and family's. My assessment coming away from that was 20k, the oil was still usable, 30k you should really start considering changing and at 40k you were well past due. Now I do put heavy belief in filters and replacing those every so often. These were all gas vehicles that eclipsed 200k on the odo, which I believe is nearing the end of a gas engine's life, so not much, if anything, is lost on the back end. So for a regular, run of the mill gas vehicle...my OCI is typically in the 15-25k range, depending on driving conditions/environment. Now the innernet typically crucifies me for that, but it is what it is.
 
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rcalexander105 said:
This can't apply to diesels either. My oil change interval would be around the 11k mile mark. o_O

Leave it to an engineer to try to overcomplicate something because science, and then it not work in the field. :flipoff2:

Sticking w/ my every 5k on the 5k
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To each his own. Sounds about right to me. I change mine every 10k, and it's still oil when it comes out.
 
rcalexander105 said:
This can't apply to diesels either. My oil change interval would be around the 11k mile mark. o_O

Leave it to an engineer to try to overcomplicate something because science, and then it not work in the field. :flipoff2:

Sticking w/ my every 5k on the 5k
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Youd be surprised to see the oil change interval of an over the road tractor that will get 1mil plus miles before an overhaul... It's more than double that.
 
I change my oil every year, whether it needs it or not. :D

Seriously, I do 10K or 1 year with my vehicles that are not under warranty, with full synthetic and quality filters. Warranty I do recommended intervals and let the dealer handle it.

My under warranty vehicle is the only on that typically gets more than 10k/year. The other stuff typically gets changed somewhere around Thanksgiving/Christmas each year.
 
rcalexander105 said:
This can't apply to diesels either. My oil change interval would be around the 11k mile mark
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The standard duty interval recommended by Cummins for the 5.9 and 6.7 is 15k miles. Severe duty is 7500.
 
With the miles on my truck and the fact that I’m towing the maximum load it’s rated for, coupled with the data from the last 3 oil analysis, my truck is much happier at 3k-4K oil changes. As @rcalexander105 said it’s cheap insurance. Cost me $35 for fresh oil/filter and peace of mind. I’ll take it.
 
shawn said:
The standard duty interval recommended by Cummins for the 5.9 and 6.7 is 15k miles. Severe duty is 7500.
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And for the 6.0 PSD it's 7,500 and 5,000 for severe duty.

For a 15 y/o engine w/ 200k on it...I'll change it at 5k. Esp since that's all the miles I put on it last year. I see no need to stretch that out. Easy to remember and relatively inexpensive to change.
 
shawn said:
Incidentally, that's also the recommended injector replacement interval.
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Coincidental that these two are interrelated...?? Now you're catching on to why I change the oil at 5k

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LBarr2002 said:
I change my oil every year, whether it needs it or not. :D

Seriously, I do 10K or 1 year with my vehicles that are not under warranty, with full synthetic and quality filters. Warranty I do recommended intervals and let the dealer handle it.

My under warranty vehicle is the only on that typically gets more than 10k/year. The other stuff typically gets changed somewhere around Thanksgiving/Christmas each year.
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This is what I do to. I don't even think my 15 Cummins lists a severe duty interval, I think it's just 15k across the board. It's rare for me to get 10k a year on a my truck though, so I change it and the RamCharger every spring before the first wheeling trip. Wife's car goes every 5k to keep the warranty people happy, but even then the oil life meter is only at 30-40% oil life left.

Duane
 
I changed the oil for the first time in 4 years in my K30 a few days ago...:D whoops...

I run 25k or so in my Duramax. Usually send in an oil sample every other time.
 
You kinda have to change it regularly on a HEUI injection engine. Lots of sheering going on with the oil and it gets worked pretty hard.

That is...unless you're @rockcity ...right? :D How long did your old 6.0 PSD go without an oil change?
 
Croatan_Kid said:
You kinda have to change it regularly on a HEUI injection engine. Lots of sheering going on with the oil and it gets worked pretty hard.

That is...unless you're @rockcity ...right? :D How long did your old 6.0 PSD go without an oil change?
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I know it was beyond 40k miles
 
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