Which Tranny & Dif Lubes?

[DaveinSoCal]

I've read through many threads but am still unsure what to use for my application so I thought I'd ask you guys.
My truck is a '98 Dakota with a 2.5L (4 cyl and a 5 speed stick which the dealer says should be an AX-15. The truck has 103k miles on it and the trans and dif lubes have never been changed.
The manual says to use 75w90 API GL5 Gear Lubricant in the transmission, and 80w90 Multipurpose Type GL-5 Gear Lubricant in the dif. Aren't they basically the same product except for the 75w/80w part, or is there a difference? And what's the difference between "Gear Lubricant" and "Multipurpose Gear Lubricant"?
Why does Chrysler not recommend changing the Dif lube; is it a good idea to change it anyhow? Also says to fill 1/4" below the fill hole if it's an 8 1/4 axle but I don't know what that is.
Am planning to use synthetic in both, any problem there?
What lubes would be best for these applications - Amsoil, Redline, RP? And what about the regular synthetics like M1 or Valvoline?
A lot of questions here I know, but thanks for any help you can give me. Dave

 

[Donald]

If you are going with Amsoil, look on their website they have an application guide where you can enter your exact vehicle.

An 8 1/4" axle refers to the diameter of the main gear in the diff. Unless you take the diff apart you cannot easily measure it, so the dealer should be able to tell you from the VIN or some other code.

 

[DaveinSoCal]

Donald, thanks for your response. I checked Amsoil's site and it shows 2 Manual trans lubes, neither of which appear to be correct for my application. One is for the trans that comes in the Dakota with the bigger motor, and the other is a GL-4. Neither are shown as a replacement for Chrysler part #4549624 which mine requires.
To further confuse me the site says "AX-15....GL-3"; my manual says GL5. Please keep the answers coming guys. Thanks.

 

[Steve S]

Redline or Amsoil makes good oils Shaeffers makes real good gear oils, Delo ESI is one of the best non synthetic oil which out performs many if not all syn oils. If you have a non slip posi type just get Amsoil or Redline as no limited slip additive is needed.

 

[unDummy]

Since you require a GL5 gear oil for the manual transmission, use the Redline 75w90NS, Amsoil AGL, or the Motul Gear300. I also would think that the GL4's like MT90 and MTG would work perfectly too.

Any full synthetic 90wt GL5 gear oil will work well enough in the diff's. I wouldn't worry about the 'w' number for the ends because of the location.

Maintenance on a vehicle is always good.
Factory intervals and recommendations usually are a joke.

 

[Pablo]

I agree - it is a bit confusing. While you certainly don't want to use a fully loaded GL-5 75W-90 in that gear box, a GL-4 75W-90 will do no harm and will actually offer some protection. The main problem is that GL-3 is no longer a current API classification!

http://www.api.org/certifications/engineoil/pubs/upload/1560.pdf
http://www.lubrizol.com/products/automotive-gear-oil/J306.asp

So for the transmission:Amsoil GL-4 75W-90 MTG

For the diff: Amsoil 75W-110 SVT or you could step up to the 140 for a limited slip.

 

[Gary Allan]

Quote:

---

The manual says to use 75w90 API GL5 Gear Lubricant in the transmission

---

This should not be correct. My 99 TJ owners manual states GL5 for the AX15 ..but it's a misprint. It requires GL3 (good luck finding a GL3). The MOPAR synthetic gear oil specifically for this application is about $15/quart at the dealer.

You can use GL4 with an M rating ..but since I don't know the visc of the Mopar stuff (I do not believe that it is matched in OTC fluids) ..I don't know if your shifting will improve or not in colder weather.