Gear oil 80W-90 versus 75W-90

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So you want to say that GM specified AMSOIL?
If you look on my table on first post you will see AMSOIL 80W-90 there. But I know of no factory fill synthetic 80W-90 like you stated.
Yes, Chevy, GM even Ford have 75W140 synthetic rear axle, but that's not the subject of this thread.
Not at all. I wasn’t referring to to factory fill, just to the fact that 80w90 does come as a 100 % synthetic in some brands. What is the subject of this thread? Your title doesn’t say anything about factory fill.
 
So in the first post I have shown the ONLY synthetic 80W-90.

Then you come and say... "warning some 80W-90 come in synthetic". Well.. no news there. Then you bring about Chevy blablabla like examples of synthetics.
So I ask what synthetics are those in your example... and you go on a tangent that ends up with "Your title doesn’t say anything about factory fill.".
OF COURSE IT DOES'T. I didn't bring that into discussion, you did:
Be careful. 80w90 comes in both synthetic and non synthetic. I think that is more important than 80w90 vs 75w90. Chevy 1/2 tons spec conventional in the front diff. Chevy 3/4 tons specify synthetic in the front diff. Both spec synthetic in the rear diff. (at least this is how is was with the mid-2000 pickups. )
In the end it's not that relevant if is synthetic or not, as long as it respects the viscosity points of the 80W-90.
My fist table shows that clearly.

I don't get what's the point that you try to make... I am confused. That I did a bad job with the table?
Fine, more power to you, you are free to make your own.
 
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My 2015 F150 manual specifies 75w-90 so I put in Mobil 1 75w-90 full synthetic. Now Ford is back specifying 75w-140 for all the axles, but I am not going to go through the trouble. 80w-90 is just fine too.
 
Now Ford is back specifying 75w-140 for all the axles
My Explorer with V8, AWD, for the 8.0 LS rear diff had a tag that read "75W-140 lifetime synthetic". Of course that's on normal usage, but no car is normal usage IMO.
For the front had 80W-90 and the LS central diff had... Mercon.
 
So in the first post I have shown the ONLY synthetic 80W-90.

Then you come and say... "warning some 80W-90 come in synthetic". Well.. no news there. Then you bring about Chevy blablabla like examples of synthetics.
So I ask what synthetics are those in your example... and you go on a tangent that ends up with "Your title doesn’t say anything about factory fill.".
OF COURSE IT DOES'T. I didn't bring that into discussion, you did:

In the end it's not that relevant if is synthetic or not, as long as it respects the viscosity points of the 80W-90.
My fist table shows that clearly.

I don't get what's the point that you try to make... I am confused. That I did a bad job with the table?
Fine, more power to you, you are free to make your own.
Seems like things went sideways. These discussions are always best done over a beer. Hope you got what you wanted from this thread. No hard feelings.
 
I added a couple of gear oils to the table, Torco RGO 80W-90 (racing oriented) and the Amsoil 75W-110. I can't edit the first post like I can on other forums, so here it is.

gear oil.jpg
 
see a lot of people asking if their specification 80W-90 can be substituted with something else. Invariably the chorus here says "use 75W-90 is better because muah, synthetic".
Since my car differential and transfer case also requires 80W-90, I kind of thought... why Toyota din't say 75W-90 if that was so great?
To start, I measured my differential temperature after running 30 minutes in 70 F ambient. Result: 110...117F. For people in the rest of the world that's 43...47C.
I am curious... let's take a look at the viscosities of various gear fluids at this temperature. Not at frozen -50C, not at scorching 100C , but at normal temperature for this application, 40C.
Well... looking at that table I don't think at all that 75W-90 is an adequate substitute for 80W-90.
Personally I went on the Redline 75W-110 route, because that's a synthetic with 40C viscosity closer of what my car manual specifies.

View attachment 48661
My gm truck says 75w90 is not required but is optional. 80w90 was the spec. 2005 Silverado. At least for the front diff I had to top up the other day. I'm doing a quick flush with 80w90 bulk oil and then filling with synthetic 75 90 (it looked like it had a lot of water in).
 
80w90 was the spec. 2005 Silverado
Based on the above data, I would go for 75W-110 on that truck that was specified 80W-90. I see no point in lowering the viscosity at 40C (110F)... even if GM says that was "optional". Especially not with Valvoline 75W-90 that is the thinnest of the group.
But again, everyone does their own decisions.
 
Based on the above data, I would go for 75W-110 on that truck that was specified 80W-90. I see no point in lowering the viscosity at 40C (110F)... even if GM says that was "optional". Especially not with Valvoline 75W-90 that is the thinnest of the group.
But again, everyone does their own decisions.
what about a 74w111 or even a 73w112?
 
That was sooo funny. For my 7 yr old.
It was funny since you made it pretty obvious you don't know anything when you made up your own make believe grade. Lol. You're not gaining any more credibility here than you were in the transmission fluid discussion. Lol
 
Based on the above data, I would go for 75W-110 on that truck that was specified 80W-90. I see no point in lowering the viscosity at 40C (110F)... even if GM says that was "optional". Especially not with Valvoline 75W-90 that is the thinnest of the group.
But again, everyone does their own decisions.
I can go with 80w90, 75w90 (both of which are recommended specs) or 75w140 which I've been told can cause damage in colder climates. I've never seen a 75w110. Care to share a link? Lol
 
I can go with 80w90, 75w90 (both of which are recommended specs) or 75w140 which I've been told can cause damage in colder climates. I've never seen a 75w110. Care to share a link? Lol

Post in thread 'Cant Find diff & transfer case fluid 2012 RAV 4 85w-90. LiquiMoly?'
https://bobistheoilguy.com/forums/t...12-rav-4-85w-90-liquimoly.332699/post-5542132

 
I must be the only bitog member that puts 80w140 in everything. 75w will give you -40 cold weather performance. The 80W140 synthetic, typically around -32C. The channel points are likey much lower being "synthetic" and all.
A big spread like 75w140 is like putting 0W50 oil in your engine.
 
If an open differential gets much hotter than 20C over ambient, IMO there is something very wrong with it.
Based on what? My Rubicon runs between 87C and 107C in 35C temperatures. I am pretty sure that is more than a 20C difference and there is nothing "very wrong with it".
 
My Rubicon runs between 87C and 107C in 35C temperatures.
You sure that's not degree F? If that was true, to me that would be the more reason to use 110 weight.

I don't know how you measure the temperature, I did actually did measure it - the external temperature of the diff case.
That's Aluminium, so the oil might be hotter, and I was not towing, just driving around town. The picture is in F.

20210309_171547.jpg
 
You sure that's not degree F? If that was true, to me that would be the more reason to use 110 weight.

I don't know how you measure the temperature, I did actually did measure it - the external temperature of the diff case.
That's Aluminium, so the oil might be hotter, and I was not towing, just driving around town. The picture is in F.

View attachment 49313
I converted from F to C to be able to easily compare to what you were saying. In F, my rear axle temps in 95°F ambient temperatures run between 180°F and 225°F.

I measure the oil temperature directly because I have sensors mounted inside the front and rear axle covers. I run 75W-140 in the rear axle and 75W-110 in the front axle.

My point is that temperatures that are more than 20° C difference between ambient and the actual oil temperature are not a problem. Moreover, I would be skeptical if they were not, because I have never seen an axle under load operate within a 20° delta from ambient.

Last, but not least, I would use 75W-110 instead of 75W-90 or 80W-90 because 75W-110 is the upper viscosity that 75W-90 used to be before they updated the J306 tables a while back.
 
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