Selecting GL-5 Gear Oil Viscosity for Motor Home Differential

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BHopkins

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I am preparing to change the differential gear oil in my 2001 motor home, with 97k miles. I got no service records when I bought the motor home in Dec. 2021, so I'm anxious to get fresh fluid in the diff.

The service manual for the rear diff (Meritor) includes 5 different GL-5 multi grade viscosities, ranging from 75W-90 to 85W-140. Beyond which viscosity to use in expected outdoor temperatures, the manual makes no recommendation of one of these grades over another. Any of the listed grades will work within the climate that I plan on driving the motor home in, so no issues there.

I like the idea of running an 80W-90 in hopes that it will help a little with fuel economy. But would and XXW-90 offer enough protection for the diff in a rig with a 32,000 GVWR? If a XXW-90 isn't enough to protect the diff, then why would Meritor include them in the service manual? So then I question if I should be selecting one of the XXW-140 gear oils, to give more protection.

Any advice from those of you out there with experience operating and maintaining heavy equipment?
 
Nearly every new OTR truck is running 75w90 in the differential. It will work fine in your application. With that said, the limited mileage that most motorhomes get will negate any economy difference. I'm stuck in the old school mindset with gear oil that thicker is usually better and in my heavy trucks I use 75w140.

Here's an excerpt from Fleet Equipment Magazine discussing this exact topic. An interesting side note is that fleets have largely moved away from 50wt transmission oil and are now using 40wt in manuals.

"The use of 75W-90 gear oil is recognized by OEMs as a contributor to overall fuel economy. When run under real-world line-haul conditions, the use of 75W-90 can improve the efficiency of a Class 8 axle by an average of 0.5% over an axle using 80W-90. Considering that axles themselves are 95% efficient, a gain of 0.5% is significant."
 
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You will never see the "fuel economy" gain using a thinner lube in such a huge rig; it would be imperceptible.

Use anything from at 90 to 140.

Use this list to choose one you like; these are all qualified to the same performance standard.

Gear oil ... PRI QPL

This is a link to the PRI website. Under the Documents section, you'll find the QPL (Qualified Product List). Any lube on that list is licensed to the performance standard. They do a good job of updating the list; I believe it's done quarterly, so this link should be current as long as you don't...
bobistheoilguy.com
 
Thank you, @Creekside and @dnewton3. I really appreciate the excerpt and the link. And particularly, the experienced advice.

Based on your advice, I feel good about going with any of the synthetic options, so I can extend my oci, based upon Meritor manual recommendation.
 
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FWIW I run Mystik 75w90 synthetic lube in my Ram that pulls our TT. I like the price and the lube seems to work good.

Just my $0.02
 
We had over a dozen class 8 trucks at work along with 5 or 6 class 6 and a number of GM trucks and ran Mobil 1 75W90 in every one of them. In the 14 yrs I was involved with their care we never had any issues and EVERYTHING about how we used them was considered severe service.
 
jetman said:
I've been running mostly Amsoil or Mobil 1 synthetic 75w90 in the 5 tag axle diesel pusher MHs that we've owned in the last 20 yrs. No issues. Our current MH (last 10 yrs) is a Prevost, quite heavy. Here's a post w/UOA I did here at BITOG last time I changed its rear diff gear oil about a yr ago:
https://bobistheoilguy.com/forums/t...d-my-sample-for-analysis.360658/#post-6292802
Click to expand...
64bawagon said:
We had over a dozen class 8 trucks at work along with 5 or 6 class 6 and a number of GM trucks and ran Mobil 1 75W90 in every one of them. In the 14 yrs I was involved with their care we never had any issues and EVERYTHING about how we used them was considered severe service.
Click to expand...
@jetman and @64bawagon, when I do a search for Mobil 1 GL-5 75W-90, most of the responses I see are for Mobil Delvac 1. Is that what you are using?

@MolaKule, I like your suggestion of the 75W-110, but the only name brands that I could find it in, are Amsoil and Red Line. If I could get them for a competitive price, I think that is what I would use.
 
Mobil 1 Delvac 75W-90 PDS

PdsDetailsPage

Mobil Delvac 1 Gear Oil 75W-90 is a fully synthetic drivetrain lubricant engineered to meet very demanding extended drain and warranty requirements.
www.mobil.com www.mobil.com
 
BHopkins said:
@MolaKule, I like your suggestion of the 75W-110, but the only name brands that I could find it in, are Amsoil and Red Line. If I could get them for a competitive price, I think that is what I would use.
Click to expand...
I found some 75W110 at O'reilly's , I believe it was a house brand and semisynthetic, but hey.
 
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