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Old 10-24-2012, 09:16 AM   #1 (permalink)
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Default 75/90 or 80/90 gear oil

I want to change the tranny fluid in my T-10 four speed and the rear end oil in my 8" rear axle.

What is the difference between 75/90 and 80/90 gear oil? Is there a signifigant gain from one versus the other?
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Old 10-24-2012, 10:56 AM   #2 (permalink)
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Originally Posted by Jays66gt View Post
I want to change the tranny fluid in my T-10 four speed and the rear end oil in my 8" rear axle.

What is the difference between 75/90 and 80/90 gear oil? Is there a signifigant gain from one versus the other?
There won't be any performance gain with either one that would even be measurable - the 75/90 would flow a tiny bit better in the freezing cold weather if you drive the car in the winter might make shifting a little easier first thing after startup - once the trans warms up they should be comparable.
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Old 10-24-2012, 04:48 PM   #3 (permalink)
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I'm doing my rear end as well, what about Syn. vs. Dino oil?
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Old 10-24-2012, 04:52 PM   #4 (permalink)
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Good question Lynn. I was thinking dino oil, no synthetic for me. But I'll be curious to the responses...
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Old 10-24-2012, 06:01 PM   #5 (permalink)
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What you want is a 75W-90 or 80W-90 GL-4 spec in the transmission. Dino or Synthetic is acceptable although some say synthetic is too slippery for the synchro's...

If have a conventional axle you want 75W-90 or 80W-90 GL-5 spec in Dino or Synthetic. If you have a limited-slip axle you can use the same or use a heavier-duty 75W-140 hypoid gear lube also in GL-5 spec. If you have a limited-slip axle with a clutch-type diff also add a friction modifier for limited-slip applications to prevent chatter.
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Old 10-24-2012, 06:28 PM   #6 (permalink)
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For over 10 years/50,000 miles I've used both Mobil 1 75w-90 and Mobil 1 75w-140 in the LS rear end and transmissions ( T-10 and a toploader) of a '66 GT350 recently sold. Fairly severe duty with many high speed endurance runs of over 100 miles each. No leaks, no measurable wear, and no shifting issues. Any high quality synthetic oil is going to be a major improvement over conventional oil in the friction reducing department.

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Old 10-24-2012, 06:36 PM   #7 (permalink)
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Quote:
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What you want is a 75W-90 or 80W-90 GL-4 spec in the transmission. Dino or Synthetic is acceptable although some say synthetic is too slippery for the synchro's...

If have a conventional axle you want 75W-90 or 80W-90 GL-5 spec in Dino or Synthetic. If you have a limited-slip axle you can use the same or use a heavier-duty 75W-140 hypoid gear lube also in GL-5 spec. If you have a limited-slip axle with a clutch-type diff also add a friction modifier for limited-slip applications to prevent chatter.
What is the GL-4 or GL-5?

I was thinking of this for the T-10 and rear:
Coastal/1 qt. 80W-90 gear oil (12401) | Gear Oil | AutoZone.com
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Old 10-24-2012, 06:42 PM   #8 (permalink)
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What is the GL-4 or GL-5?

I was thinking of this for the T-10 and rear:
Coastal/1 qt. 80W-90 gear oil (12401) | Gear Oil | AutoZone.com
what is point of putting the cheapest oil possible in your car?

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Old 10-24-2012, 06:48 PM   #9 (permalink)
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Does spending another $1.00 per quart to go to Valvoline get me anything more?
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Old 10-24-2012, 08:54 PM   #10 (permalink)
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Does spending another $1.00 per quart to go to Valvoline get me anything more?
I'm not in the Valvoline camp, so I can't really speak to that. But in my opinion spending double the money on any quality synthetic is a bargain.

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Old 10-25-2012, 04:27 PM   #11 (permalink)
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Based on BartL's post above and my limited experience, I was leaning toward conventional. I've no experience with synthetics. None, zero.
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Old 10-25-2012, 08:03 PM   #12 (permalink)
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Based on BartL's post above and my limited experience, I was leaning toward conventional. I've no experience with synthetics. None, zero.
before your first breath you had no experience breathing. Once you started breathing you quickly got the hang of it, and now probably think it's pretty cool.

Same with using synthetic oils.


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Old 10-25-2012, 11:19 PM   #13 (permalink)
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I was only speaking from what I have heard/read. I have had no issues with synthetic lubricants either.

The difference between GL-4 and GL-5 is the amount of phosphorus in the formula. GL-4 has about 1/2 the phosphorus as GL-5. In a typical rear axle with steel gears, the phosphorus forms a black sacrificial layer between the gears to provide protection. That's a good thing and the steel is harder than the phosphorus so it peels right off afterward. In modern manual transmissions, with brass synchros, the phosphorus forms the same black sacrificial layer but the bad is that its HARDER than the brass, so each time the sacrificial layer is peeled away it takes a micron or two of the brass along with it. Over time you can guess what happens.
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Old 10-26-2012, 12:56 AM   #14 (permalink)
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Quote:
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"................"but the bad is that its HARDER than the brass, so each time the sacrificial layer is peeled away it takes a micron or two of the brass along with it. Over time you can guess what happens.
the key here is the "over time" wording. Over a period of ten years of severe usage I experienced zero wear of the gears and synchros when using the GL5 rated Mobil1 gear lube.

Over a period of ten thousand years what you might find worn might be another story, but of not too much concern to me.

Z.
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Old 10-26-2012, 08:16 AM   #15 (permalink)
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Not gonna get into the synthetic vs mineral oil controversy, but I would like to share something about a test I did in the '90s with rear gears. I drove my 1965 Falcon w/8" single track gears running GL5 90 wt mineral oil on a 150 mile trip in mid summer (high '90s). Measured the heat on the case, then drained and refilled with synthetic oil (don't remember the brand). Drove the car 150 miles and measured the heat again. Not one degree of difference. Whatever friction change that took place could not be measured by the heat difference.
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