Another axle/transfer case fluid question (sorry)

So on my last post asking about these fluids, different weights were recommended for each.
One said 80w90 for the transfer case, and 75w90 for the axle.
Another said 75w90 for the transfer case, and 80w90 for the axle.
And yet another said 85w90 for all of it.

FSM just says GL-5 or higher for the front axle and 75w90 for the transfer case.

Is there a reason for 80w90 vs 75w90?
It's a higher mileage truck at this point so I'm inclined to go heavier, but just trying to be sure.

Sorry for all my questions. I try to find out as much as I can about anything, especially when trying to take the best care of this car.
 

Inyo_man

Don't piss down my back and tell me it's raining.
Below is lifted from:

Gear oil is used as a lubricant between the moving parts of a gear system so that the gears mesh and turn smoothly. Depending on the type of gears and the operation conditions the gear oil will need to possess different physical properties to adequately lubricate and protect the gears.


Viscosity
The measure of the thickness of a liquid is called viscosity. Though there are several different measurements for viscosity, the one standard with gear and other automotive oil is the Society of Automotive Engineers (SAE) weight classification. The larger the SAE grade number the more viscous the oil.

Temperature
Viscosity is affected by temperature. SAE viscosity ratings are given for the oil as measured at 212 degrees Fahrenheit. As oil gets hotter it becomes less viscous, as it gets cooler it becomes more viscous. Imagine putting honey in the microwave; at high temperature it flows more quickly. For cold weather oil the SAE has a "W" certification for oil that is tested at 32 degrees Fahrenheit. The grade will be indicated with a number followed by the letter "W" to indicate winter viscosity.
Grade
The SAE grading system indicates oil viscosity in simple numbers ranging from the thin oil (10) to the very thick oil (140 or above). Heavier weight, or higher grade oil is more viscous and therefore thicker as it flows between the gears. Light grade oil is ideal for small, fast, polished gears under a light load. Heavy grade oil is suited for large, slow, rough gears under a heavy load. Check your manufacture's specifications for the gear grade needed in your equipment.
Multi-Grade
To account for changing temperatures oil is manufactured through the use of additives so that it can have multiple grades of viscosity, or multiple weights. One weight will be a winter weight and the other will be the hot weight. So you may see gear oil listed as "75W-90" which gives you both the winter and hot weights of the oil. Instead of switching out the oil during the year to compensate for the changing temperatures, most equipment will allow a multi-grade oil to be used year-round (check your manufacturer's specifications to be sure).

I prefer Amsoil, but there other choices...

Diffs.:

Transfer Case:

I'd also suggest getting a pump for filling.

Cheers
 

shade

Well-known member
I'd also suggest getting a pump for filling.
I've used the fiddly pumps that screw onto 1qt bottles, but for moving more fluid, I use a cheap garden sprayer for ATF and gear lube. Remove the adjustable tip from the wand for more flow, or just cut the wand for maximum flow. I suppose the wand could be removed altogether leaving the hose, but without a valve, it would no longer be possible to pressurize the sprayer.
 
in previous years the viscosity variances were due mainly to manufacturers spec, each special a different oil for proprietary reasons..in relaity a lighter grade for cold clime and ...a lighter grade for warm climes too.

there are other reasons for vis. grades unimportant to this discussion.

generally only one bit of advice with gear oil.
Use synthetic.
the synthetic marketed under Wal Mart brand is a very good gear oil. it punches way above its weight class, in testing.

the specialty oils are far too expensive and their added benefit for our purpose is wasted. if ur hellscat whoosh, model 755 interstellar race car that turns 17000 rpm at the 24 hour Lemans, then a specialty blend is required.

the basic synthetic must pass the same tests to meet spec. trust it, save a bunch a dough, and take ur honey to dinner. u will feel better about it both ways and ur car don't give a dam either way.

what is important is it's shear factor it's EP rating, and the fact that synthetics will save u a lot of money through increased efficiency and reduced rotating resistance.

if u have limited slip then purchase oil with that additive in it.

a tube generally does 4 quarts, if u want to add ur own l.s additive
this is an area where manufacturer or brands might make a difference.
otherwise the sta-lube, tube is just fine.
rock this scene with practical intellugence. you can way overspend and overthink this part of ur maintenance schedule.

last, engine oils do not follow the same rules
in ur mitsubishi engine, a Japanese oil like eneos or eqiv. really is a must, stretch the changes to 5500 miles or so with filter changes at 2000 or so.
Japanese metal loves Japanese oil. there is no good alternative to this edict.
 

coffeegoat

Adventurer
Just a quick word of warning, switching to synthetic is great for your gears but it may cause a seal leak. The mechanism isn't well documented (at least that I could find) but the general thought is that a existing leak could be clogged by gummy old conventional oil but the fresh new synthetic flushes out the crap and starts to leak badly. It happened in one of my front axle seals and is annoying, but I guess a new seal is cheaper than new gears. Furthermore, it's not clear if it is really related to synthetic, rather just "new" oil, but since most people replace with synthetic there are more cases of it "causing" a new leak.
 
I appreciate all responses, and I'm already familiar with how these are rated, but what I was trying to get at was: I have 244k on the odometer. I live in Middle TN where we do get some cold winters, but it's not usually lower than about 15 degrees for very long (at least that I've seen in 6 years).
Which weight would be best in this application? I also have no problem unbolting the front diff to clean up the seal before filling it, though I can't seem to find if there's a gasket in there or if using a liquid gasket/seal will work just fine.
 
new lube will have detergents that old lube no longer has.
ingredients in lubes are detergents,
,anti friction compounds etc.
the oil does not break down, but these additives do. thus fresh detergents will open up old wounds.
lsd additive is good for about 2 years more or less.
 
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