Cummins N14 5W40 vs 15W40

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I have a 1999 Monaco Signature 45’ Class A. The motor is a Cummins N14. Just added a FS2500 bypass filter plus the factory Fleetguard FF filter. I haven’t filled it with oil yet, but want to run synthetic and do EOC. Any issues with running 5W40?
 
Probably not, but in Arizona, no need to. The 15w40 would be just fine.
There is no advantage to synthetic unless the conditions call for it. Temperatures or extended drains, maybe, but there are many, many UOA that prove that a conventional will do just as well as a full synthetic, depending on application.
 
Neither of those are available in my town. My only synthetic option is the Rotella T6 5W-40
 
Many millions of miles logged on the old N14 engines using nothing but conventional 15w40. Spend the money on clean air filters and clean fuel filters as well as coolant.

How much of an extended drain do you want? 10000 miles is the normal change interval.
 
My plan was to go as long as possible using analysis. I figure with the bypass filter I should be able to easily go 20,000 miles. That was based on my reading regarding synthetics since they have a higher TBN
 
Higher starting TBN doesn't necessarily mean it lasts longer though. TBN can drop faster or slower depending on oil and conditions.


If your only synthetic option is T6 in 5w40 then run it if you are dead set on extended drains. You don't need it for the winter weather starting obviously but I would feel more comfortable with the synthetic going 20k miles with the bypass filter setup.
 
If you're attempting extended drain intervals then yes you should use synthetic, and you should being that you've went to the expense and trouble to install a bypass filter setup.How many gallons of oil does the N14 hold?

I'm sure that ol Cummins will love the Rotella 5W40 either way.
 
The only advantage the 5w- will buy you is below freezing starts. If you can use the starting advantage then run the syn.
 
I can’t find anyone who has run the 5W40 in that engine, hence my concerns. The manual calls for 14 gallons.
 
Despite being in Arizona, I don’t live at the lower elevations. We were freezing up until about a week ago. I live at 8000 FT ASL
 
I’d love to go that long! I think with UOA and the bypass filter, I’m hopeful. I hate wasting good oil just because it’s been. Year. I don’t think I’ll put more than 20k per year anyway. Mostly just hate wasting 14 gallons of oil when I don’t need to
 
Originally Posted By: offtosleep
I can’t find anyone who has run the 5W40 in that engine, hence my concerns. The manual calls for 14 gallons.


A lot of people have ran 5w40 HDEO in their N-14's, 350 engine hours should be easy to do with just about any good quality HDEO......That's a lot of run time for a recreational vehicle!!
 
Originally Posted By: A_Harman
Just run 15w40.


^^^ TRUTH! ^^^

Really, any (yes, any) 15W-40 that meets specs will be fine.

CHECK YOUR COOLANT chemicals regularly!
Big Cummins diesels will run forever and ever on the cheapest 15W-40
that meets API specs that money can buy....

but bad coolant will eat them up fast.
 
Originally Posted By: offtosleep
I hate wasting good oil just because it’s been a year.


Then don't change it.

A $20 oil analysis kit is a LOT cheaper than 14 gallons of synthetic oil!!!!!!!
 
T6 will take you to 20K with by-pass easy as pie. No worries
smile.gif
 
I did a lot of my Mechanic apprenticeship at a fleet consisting primarily of N-14 engines, this would have been back in the CI oil quality days. Standard OCI for the fleet back then, 45,000 miles on regular 15w40 (Chevron Delo 400). Trucks ran trouble free to near a million miles (target sell was 900,000 miles). N-14 failures were usually coolant or fuel related, never oil related.

Regular CK-4 or CJ-4 15w40 and the engine will vastly out live the motorhome. Do a simple oil sample every 10,000 miles if you are worried (a good sample test like Polaris or Blackstone...not a table top unit at a Speedco or truck dealership) but you should EASILY be able to go 25,000 miles on regular oil. The N-14 is not hard on oil AT ALL!
 
Few things:

Oil can't read a calendar so my advice is forget the "one year" stuff. If the MH is stored well in a relatively dry climate, used regularly and when used driven enough to boil off crankcase moisture, the oils is good out to several years IMO. That opinion is based on operating a variety of equipment that never reaches the miles or hours limits within one year. Over the last 15 years or so I have UOA at one year, two, years, three years and found no issues. My limit is set at four years but seldom does anything get out that far except for one truck, which is due for a change this year after four years and about 5,000 miles. Truck is an old school IDI diesel with bypass filtration.

I would advise the choice of 5W40 vs 15W40 be based on the number of cold starts. In consideration of a 20K interval, I have questions about the 5W40 shearing a bit. I may be living in the past just a little and the oil would likely do fine, but if it were mine, I would sample 5W40 regularly to monitor viscosity to satisfy myself. Maybe you can turn up some trustworthy evidence out there in posted UOAs. Back to the number of cold starts, if you have to start the MH regularly in cold weather, that's when the 5W40 is more indicated because cold starts with thick oil tend to be the ones with more wear, more filter bypass events, etc. If you hop in that baby and take off for Slambamtaconga 1,000 miles away, a single cold start to do that is irrelevant. If that's how the MH is operated, the 15W40 is the way to go IMO. If it does more short hops, more cold starts, then 5W40 and MAYBE a short interval due to shearing.

If you are set up to sample, you can conduct your own test. Use 5W40, test every 5K (or one year) or so. Then you will know ... and learn something on the way. If the 5W40 doesn't hold up, you can switch to 15W40 with confidence.

Finally, look up what oils the latest version of the N14 were authorized to use. That is always a good indicator as well.

The N14 was built to 2002 and I have seen the last N14 engine that rolled off the line. At the same time, I saw one of Cummins' earliest Model H engines of 1934, which is the direct core ancestor of the N14. Am doing a story for Diesel World magazine on this evolution. What a fantastic run and at 1999, your engine has 65 years of evolution behind it!
 
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