Which is the best ol for an 80’s 7.5?

[Mike Tonon]

Well, which oil and why?

 

[Dirtman]

Gear oil.

 

[Mike Tonon]

Gear oil.

They’re all gear oils.

 

[Dirtman]

Yes, yes they are.

If you have a limited slip you need a friction modifier. Other than that it does not matter. 80w90 non synthetic worked for over a century and is what the owners manual in that truck would call for. 80w90 didn't magically stop working 5 years ago when ford said it recommends 75w140 synthetic in everything. You're talking about a 30 year old truck axle, not an F1 racecar. You will not be able to see any difference no matter what you put in it. Buy whatever is on sale, change it when it needs it. Don't overthink things. If you wanna spend 20 bucks on a quart of gear oil because it gives you a warm fuzzy feeling, more power to you, but rest assure your axle doesn't care.

 

[adsm08]

Yes, yes they are.

If you have a limited slip you need a friction modifier. Other than that it does not matter. 80w90 non synthetic worked for over a century and is what the owners manual in that truck would call for. 80w90 didn't magically stop working 5 years ago when ford said it recommends 75w140 synthetic in everything. You're talking about a 30 year old truck axle, not an F1 racecar. You will not be able to see any difference no matter what you put in it. Buy whatever is on sale, change it when it needs it. Don't overthink things. If you wanna spend 20 bucks on a quart of gear oil because it gives you a warm fuzzy feeling, more power to you, but rest assure your axle doesn't care.

^This^

I know a guy who just puts straight Lucas Stabilizer in his axles. He has two dedicated wheelers with various Dana axles and none of them have issues.

 

[Mike Tonon]

Yes, yes they are.

If you have a limited slip you need a friction modifier. Other than that it does not matter. 80w90 non synthetic worked for over a century and is what the owners manual in that truck would call for. 80w90 didn't magically stop working 5 years ago when ford said it recommends 75w140 synthetic in everything. You're talking about a 30 year old truck axle, not an F1 racecar. You will not be able to see any difference no matter what you put in it. Buy whatever is on sale, change it when it needs it. Don't overthink things. If you wanna spend 20 bucks on a quart of gear oil have at it if it gives you a warm fuzzy feeling, but rest assure your axle doesn't care.

Let’s say this truck is a mainly factory, resto-project and I rebuilt the axle. Let’s say it’s a Supercab, with the 2.9 engine, 2wd and will be used for commuting and light duty travel 95% of the time and loaded up and almost abused 5% of the time. Let’s say it’s in a location where temperatures range from -10F to 105F. And let’s say it’s a 7.5 open differential, with 3.45 gearing. Which is the most adequate oil? (Forgetting price, because if I spend an extra $20 or about that on it and don’t change it for 100,000 miles or more, who cares.) Or should it be changed after maybe 15,000 miles from rebuilding the axle and then after no less than 100,000 miles?

Thanks, you’re a real pal!

 

[alwaysFlOoReD]

Price no object? Use the Ford recommended 75w/140 full synthetic.

 

[Mike Tonon]

^This^

I know a guy who just puts straight Lucas Stabilizer in his axles. He has two dedicated wheelers with various Dana axles and none of them have issues.

I’ve known people with 7.5 and 8.8 axles that started leaking. I’ve know people with those axels that started making noise and were told that they needed a rebuild (several hundreds of dollars, into the 1 or 2 thousands of I remember right) or they could try swapping in a junkyard axel. I’d rather avoid these scenarios by any way possible.

 

[Mike Tonon]

Price no object? Use the Ford recommended 75w/140 full synthetic.

Possibly the best bet. But for sake of dissecting the subject and learning, I wanted to discuss this. What if I used 75w90 full synthetic? Is that too slippery, too thin? Would I gain maybe .15 miles per gallon improvement on gasoline consumption?

 

[alwaysFlOoReD]

I don't know much about the subject. If you want to dive into a rabbit hole, check out

https://www.bobistheoilguy.com/

 

[Mike Tonon]

Thanks

 

[Mike Tonon]

I talked with an axle rebuild shop. He said he just uses store brand 80w90 in most anything he rebuilds. He said synthetic 75w140 has a longer life, but doesn’t seem to cling to the gears as well.

I don’t know if this is related, but in the freezing cold, I was hearing a whining noise during acceleration coming from the front of my truck. It’s never done it now that it’s warm out. I put 75w140 synthetic in there years ago.

So, I went and bought Lucas 80w90 conventional and am planning on just doing both front and rear axles soon.

Thanks for all the input though, everyone!