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Old 04-18-2016, 04:24 PM   #1
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15W40 in differential?

Here is something that just rolled me backwards when I found it in the service records of my newly acquired 1999 International 3800 conventional with a DT466E and AT545: they are using 15W40 MOTOR OIL in the rear end. Same stuff they use in the engine. I kid you not. And they were changing it every 10k miles like clock work. Record after record says the same thing, in different handwritings, over a 10 year period so I know its not just a typo.

Is this a standard practice in busses? Why not use gear lube? What am I missing here? This bus was maintained with love by a really dedicated and friendly bunch of techs, so there has to be a method to this madness.

Help? Totally confuzzified here...
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Old 04-18-2016, 06:17 PM   #2
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I know a heavy haul company here that runs 15w40 in his diff's and manual trannies. I've also seen 15w40 used from the factory in some automatic transmissions on loaders.
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Old 04-20-2016, 03:35 AM   #3
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I bought a MT647 transmission from another member here. What I thought was burn trans fluid turned out to be 30 weight or something similar. I called the bus barn and the guy confirmed it.

Did your bus come from a cold weather location?
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Old 04-20-2016, 08:17 AM   #4
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Interesting!

The bus hails from Marble Falls, Texas. Heat and humidity would be the big issues here, not so much the cold. In fact, I dont think it ever drops much below 40F, and even then, not for long.

I'm still scratching my head on this one, but I'll drain the diff this weekend and put a high-quality gear lube in it. It may be that with such short service intervals and long periods of stopping/idling they were able to get away with it. If it works for them, great, but I dont want to be stuck on the side of the road in Death Valley on an August day!

Thanks for the replies!
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Old 04-20-2016, 10:31 AM   #5
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WHAT!?...motor oil in the rear axle? --- No way. My Dana duallie requires 75-90 for moderate service and 75-140 for heavy duty service. Going with synthetics (which I did), 75-90 is good for either service.
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Old 04-20-2016, 11:01 AM   #6
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Yeah I really don't know what to say about this. Transaxles in cars can and do use fluids other then gear oil as a lubricant. But I don't see how a 15w40 fluid can offer the shear and wear protection that is necessary in a rear axle in a school bus. But if they did it without fail, then hey, good for them. But I think I'll stick with gear oil.
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Old 04-20-2016, 11:23 AM   #7
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Way back in my SCCA racing days, and at the suggestion of a Mobil engineer, we ran 15w50 Mobil One synthetic in our manual 5-speed gear boxes (VW) with good success as long as you changed it every couple of races. The stuff sheared-down pretty fast at 170 hp and 7800 rpm shift points, but it made for very good shifts and added a couple of horses over the stock fill. So stranger things have been done... but yeah, I'm really scratching my head on this whole motor-oil-in-rear-end-bus-thing too.

I ordered some 75w synthetic fill from Lucas this AM. Have had good luck with their stuff elsewhere, so we will see how this Big Iron likes it.

Again, thanks for your replies! Its appreciated.
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Old 04-20-2016, 10:22 PM   #8
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I have no idea what is in mine, but the manual actually specs SAE-40 oil for the axle in the climate I am in. This may be due to the fact that is a school bus designed to be in slow moving traffic, not doing 65 for hours on end. It was a bit low and I added 75w-140 gear oil, but the stuff in it seemed much thinner than that. Also, I have no idea where to get SAE-40 oil, never seen it anywhere. So if / when I get around to changing those 5 gallons of stinky gear oil, I will let you know what I use, but I doubt it would be engine oil!
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Old 04-20-2016, 10:31 PM   #9
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Quote:
Originally Posted by porkchopsandwiches View Post
I have no idea what is in mine, but the manual actually specs SAE-40 oil for the axle in the climate I am in. This may be due to the fact that is a school bus designed to be in slow moving traffic, not doing 65 for hours on end. It was a bit low and I added 75w-140 gear oil, but the stuff in it seemed much thinner than that. Also, I have no idea where to get SAE-40 oil, never seen it anywhere. So if / when I get around to changing those 5 gallons of stinky gear oil, I will let you know what I use, but I doubt it would be engine oil!
Only $400 for a 55 gallon drum!
SAE 40 Weight Diesel Engine Oil - 55 Gallon Drum – BuySinopec.com

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Old 04-20-2016, 10:33 PM   #10
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Quote:
Originally Posted by EastCoastCB View Post
Man, what a deal! that would really be worth investing in with the 5,000 miles a year I average on that bus! lol
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Old 04-20-2016, 10:59 PM   #11
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Blech! Chinese straight-grade dino oil with a CF spec? About the only thing I have that would use it would be a 6/1 listeroid... And that sucker will happily run on bacon grease as both a lube AND fuel.
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Old 04-21-2016, 12:47 AM   #12
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Right from the freightliner school bus manual
*

Recommended
Lubricant Type
Hypoid Gear Oil
API Service
Classification GL–5
Ambient Temperature
Lubricant SAE
+10°F (–12.2°C) and up
*
Viscosity Grade
85W–140 0–76–A
–15°F (–26.1°C) and up
*
80W–140 0–76–B
–15°F (–26.1°C) and up
*
80W–90 0–76–D
–40°F (–40°C) and up
*
75W–90 0–76–E
–40°F (–40°C) to +35°F (+2°C) 75W 0–76–J
–40°F (–40°C) and up
Synthetic Gear Oil
–40°F (–40°C) and up
*
*
Meritor
Specification
75W–140 0–76–L
75W–90 0–76–N
–40°F (–40°C) and up
*
There is no upper limit on these ambient temperatures, but axle sump temperature must never exceed 250°F (121°C).
Table 3, Meritor Drive Axle Approved Lubricants
75W–140 0–76–M
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Old 05-07-2016, 03:33 PM   #13
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Follow-up: I compared what came out of the rear-end to some fresh lube. What was in there was definitely not 15w40. I'm 95% sure it was 75w90 or thereabouts based on viscosity. It was definitely NOT motor oil, as it had the "gear oil smell and sheen" that I associate with zinc-based additive packages. So that was a relief. And frankly, it looked darned good for being in there for 3 years (but only about 4000 miles according to the records).

As an experiment, I ran it for a week with some cheap generic 90w lube from Tractor Supply that was on sale, and then I drained it and refilled with 75w140 Mobil Delvac synthetic (the local dealer, who appears to be a complete doofus, kept giving me excuses as to why the Lucas stuff hadnt arrived. I cancelled the order and got the Delvac which the local tractor pullers swear by). Its noticably quieter now.

This lube may be a bit thicker than the optimal spec, but I've never heard of a diff hurt by running a thicker lube in hot areas. All of my driving will be in the heat, (and 90% of it through a desert in the middle of summer), so I'm happy to have the thicker stuff in there.

Thanks to all who provided comments and guidance!
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