differential fluid for towing rubicon 50 towing.

[GladiatorPilot23]

Though I agree completely with doing oil analyses on all your components to see how wear metals are trending, and you’re suggesting 3 tests to know if the oil picked was a good decision, how many miles are you suggesting between tests x’ 3 ?

I know that’s gonna take forever believe me, but I was just quoting what Lakes Speed junior typically says lol. Optimally every 15,000 miles you should change it and anytime you’ve been through water. So going off of that you’re talking about 45,000 miles on each oil so that’s a combined of 90,000 miles just to get an idea of of which one is better lol

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[Maximus Gladius]

I know that’s gonna take forever believe me, but I was just quoting what Lakes Speed junior typically says lol. Optimally every 15,000 miles you should change it and anytime you’ve been through water. So going off of that you’re talking about 45,000 miles on each oil so that’s a combined of 90,000 miles just to get an idea of of which one is better lol

Ya, lol, you knew where I was going with this. That’s when I’m sure a lot of trade-ins are done. Not for me though. I’m in it for life, which ever one gives up first lol.
I stuck with MOTUL Gear 300 75/90. I don’t have a trailer I pull so my wear metals are trending in the right direction. Diff oil changes are from 25k kms to 35k kms. I tend to keep things clean (including the locker magnets inside the diffs). No axel seal leaks for me, knock on wood.

 

[GladiatorPilot23]

One other thing I looked at when I decided to go with 75W140 in my JTM rear diff was the viscosity versus temperature chart per ASTM D341. Over on the JL forums, there was data posted by CarbonSteel on rear diff operating temperatures, showing fairly high operating temperatures: https://www.jlwranglerforums.com/forum/threads/diff-fluid-wrangler-jl.97387/

Looking at Valvoline Synpower 75W90 versus 75W140 at 100C (212F):
75W140 provides the same viscosity at 262F as 75W90 at 212F
75W140 provides the same viscosity at 212F as 75W90 at 172F
So, using 75W140 is like decreasing the diff temp when using 75W90 by 40-50F at the 'high end' operating temperature. IMO, the increase in viscosity at operating temperature is only a good thing.

But, low temperature viscosity also needs to be considered. Since I live in CT, I focused on -20C (-4F) as my low temperature point.
75W140 provides the same viscosity at 14F as 75W90 at -4F
75W140 provides the same viscosity at -4F as 75W90 at -18F
So, using 75W140 is like decreasing the diff temp when using 75W90 by 14-18F at 'cold start'.
Excessive viscosity is not necessarily a good thing at cold start. But, IMO (for my use case), the increase in viscosity from using 75W140 falls within the viscosity range for 75W90 at design low-end temperatures.

Thanks for the info! I'm no expert but here is what I think I know just from learning from pro like Lake Speed Jr.

1 thing is as you mentioned the think oil is not going to be good in the real cold. If you run 75w-85 vs 80w-140 with say Redline Oil

API Service Class	GL 5+
SAE Viscosity Grade (Gear Oil)	75W85
Vis @ 100°C, cSt	12
Vis @ 40°C, cSt	68
Viscosity Index	171
Pour Point, °C	-45
Pour Point, °F	-49
Brookfield Viscosity, Poise	300 @ -40°C

API Service ClassGL 5+SAE Viscosity Grade (Gear Oil)80W140Vis @ 100°C, cSt26Vis @ 40°C, cSt227Viscosity Index146Pour Point, °C-40Pour Point, °F-40Brookfield Viscosity, Poise500 @-26°C


The Mopar cSt looks like is 11.5 from what I found from a 2016 posting?

Amsoil 75w-85 is 13.1

If you go to 75w-140 that is over 116.67% more viscous at 100C. Who know exactly how much MPG would go down but it could be 1-2MPG possibly. Also the viscosity index is very high with these Group4/5 oils. Very stable oil even in high heat.

Take note of the VI a 171. I don't know what the Mopar is but no way this high because they are a group 3 oil. At 171 the oil is very stable in temp changes.


Amsoil is about the same specs but there oil 13cSt in 75w-85.

There is also some of the best EP additives in these oils to protect the gears which is the most important thing over viscosity.

It a hard decision and I personally would use 75w-85 from Redline or Amsoil and test wear. If I was towing full time I would atleast see the metals using that and then try the 75w-140 and test at least once but using the same top tier oil brand.

One more thing, Lake Speed did a video as I recall going from a thinner oil that was the spec to a thicker one and had more wear in the engine. He also did the opposite in a race engine. He had a thinner oil and saw to much wear and then went thicker and the wear went down a lot. This is on a high load racing engine though but the only way he knew was threw testing it.

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