I went to town the other day to buy some gear lube to change out my rear diff oil. They didnt have any strait synthetic 75w140, but had 85w140 conventional. They both show the same API service rating of GL-5. Does it really matter if I run the 85w140, as long as I fallow the service intervals for a conventional gear oil?

 

Well, I put the 85w140 in it......

 

I'm honestly confused by the gear oil too. From what I understand, you need friction modifier in there for a LSD (typically the rear diff but not the front). Some synthetics come with friction modifier. If your oil doesn't have friction modifier in it, I think you need to add some.

Don't take my word for it but I would do more investigating on this quickly before you possibly damage your rear diff with conventional oil with no friction modifier.

 

Well I do know the modifier is so the clutch packs in the LS slip easier and don't "chatter"

I just wasn't sure why I'd need synthetic in there, expecially since it has aftermarket gears and bearings in it.

 

I think Ford says to put synthetic in. Do you really want to risk it for a diff oil change that hardly ever needs to get done ?
I put in mobil-1 synthetic

 

I think that right there is why synthetic is recommended. Because alot of people just drive. Same can be said for the antifreeze used now vs the regular old green stuff.

That's why I mentioned changing over to the conventional oil service interval too. I pull heavy with this, and drop the oil yearly in the diff, needed or not.

 

sure, that sounds ok but..
Synthetic can handle a lot more stress than conventional.
Can also get a lot hotter and still maintain its function.
I don't know. how much would you save on regular v synthetic?

oh, and I am not a synthetic all guy at all. I use what ford recommends for the engine which is a synthetic blend.
But Ford does recommend synthetic for the differential.

 

I'm not looking at this so much from the conventional vs synthetic. That argument goes on for ever. I'm thinking about Ford's requirement for or suggested use of friction modifier. It stops the clutch chatter, which to me means, things will wear better. I also know you can add too much friction modifier, say for example, by adding it to a synthetic oil that already has friction modifier.

 

Guess I'm looking at it this way, I've got aftermarket gears and bearings in the diff, same gears and bearings they use from 92-04 10.25s and 10.5 rear ends. In 92 they ran 80w90, what makes this rear so special that it needs he 75w140 synthetic, expecially since it has 10.25 gears and bearings in it?

I understand synthetic has a lower pour point and what not, but, will it REALLY hurt the diff running a conventional oil? P

 

First, why is everyone jumping on the LS bandwagon when you never mentioned it.......lol
I wouldn't worry about having conventional oil in it, they have been using it for 80+- years with no problem and they have established that the synth oil will work well EXTREME temperatures but other than that the benefit is still controversial and therefore must be almost the same quality. I wouldn't worry about it myself. (Notice they both a a GL-5 rating). Also, all this hoopla about different gears and years etc means nothing, it is still just a rear end........come-on.

 

I was hoping you would see this thread.

I was thinkin the same as you, it's a rear end... What's it care whats in it. I wanted someone with some experience in this subject to chime in.


O, and I do have a LS, but that's a mook point, both oil types need the modifier in it.

 

If it's not chattering and works fine I wouldn't worry about it. They don't automatically need modifier (slippery substance) and if it does chatter it's best to just add a little at a time (not all the same day) until it quits chattering. A barely chattering limited slip is a good grabbing limited slip.

 

I never "jumped on the LS bandwagon", I was merely pointing out that if it was a LS, the OP should make sure to check about friction modifier. That's all.

 

Sorry, I just couldn't figure out why everyone was talking about an LS but I didn't read it and thought maybe I was missing something.