What Enables Such Long Intervals With Diesels??

May 12, 2019
I just read where a trucking company with the help of Mobil and Cummins have gone to a 60K mile OCI and keeps the engine in warranty as long as they do oil analysis every 10K.
Is it the fact that the 'Big Trucks" have 20-40 gallon oil capacity and the fact that diesel fuel itself is oily that allows such long intervals whereas, gas engines have much lower oil capacity and the fuel itself isn't "oily"?
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Bigger capacity, better lubrication, and also the driving conditions most diesels are used under, mostly highway miles.

Some HDEO synthetics even claim 100k OCI
I completely agree with your response! Even the old 15W40 has way more material than standard gasoline engine oil. If you went 10,000 miles on an oil change, pulled a sample, and then topped back off I'm sure it would be very easy to go hundreds of thousands of miles on very few complete oil changes. That's how amsoil signature series really shines. It's for your salesman or you're traveling nurse going long distances every day. Mobil one had their annual protection line that did something similarly but the price of it at Walmart was either too much for most customers, overcomplicated it by not making consumers still check their oil on a regular basis, or there was still the people who couldn't imagine leaving an oil in an engine for that long of a Time.
10+ gallon oil sump on most class 8 trucks, lower RPM's, lots of highway usage. My Ram only has a 3 gallon sump but the oil tolerates a lot more abuse than most any SN oil. Especially pulling through the mountains.
Applying the 200:1 fuel-burn to oil-change ratio rule of thumb:

Hypothetical truck gets 7 mpg and has 15 gallon oil sump.
200:1 OCI would be 15gal * 200 * 7mpg = 21,000 miles
It's been 25 years since I touched a Class 8 truck but the 71 series Detroit's held about 32 quarts (I think the 855 Cumming's did too) and the Cat's (3146?) held 40 to 42 quarts in the pan. Most every Class 8 truck I worked on had an engine mounted oil filter that held 2 quarts and the optional external mounted Luberfiner filter that held 9 quarts.
So there was a lot of oil filtering going on!
I also think that generally speaking a large diesel had a 15+ litre displacement and makes low power output comparatively. Lots of bearing surface and low RPM. generally cruising is between 1200 and 1800, with redline somewhere in the low to mid 2000 RPM. Less shearing, and very large sump capacity.

Also no OTR trucks I know of get over 5mpg. My BIL has a trucking company and has a few older semi-retired drivers. They play the game of getting the highest overall mileage on long hauls loaded. One guy got just over 6mpg, going 60mph most of the time have keeping RPM under 1500.
My old 1989 400 big cam Cummins held 44 qts of Rotella T4 with a 12,000 mile OCI. My last tank of fuel in 1994 was $0.899 per gallon at 6.5 to 7 mpg. My how times have changed.
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Dilution is the solution to pollution. Along with insanely precision engineered components that aren't considered disposable like most of us are used to.
And you likely never change it.
I did one a few years ago because a fuel line broke and diluted the oil. That’s how I know how much oil they take. UOA on another engine showed “critical water “. That was a cheap fix by draining water from the sump out the oil drain pipe. Locomotive engine oils don’t have emulsifiers, so water separates to the bottom.