Can someone please help me understand something? I plan on changing my front and rear Diff fluid this week. I was going to use Mobil 1 75w90 in the rear (I have a G80 Diff) I thtink it has the additive in it. I was going to go with 80w90 in the front like the manual suggest, however many say you can run the 75 in the front as well. Can anyone tell me why this is ok, and if it is why doens't the manual state that? Also, is the Mobil 80W90 sythetic or dino? I am confused and learning everyday.

Thanks in advance.

Mike

From what I have personally seen on here...everyone tends to run 75W-90. I know that I myself purchased Amsoil 75W-90 Severe Gear fully synthetic and have it in both front and rear diff's on my NBS LMM...Heath from here hooked me up!

You can use the 75w-90 (regardless of brand) up front with no issues. The Mobil 80w-90 is their conventional lubricant. The Mobil 1 75w-90 already has the friction modifiers added; just install as you would any other diff fluid.

I know the manuals are confusing. My owners manual and my Dmax supplemental manual each have a different suggested fluid for the front diff. You can "upgrade" to the 75w-90 synthetic with no issues.

The most important thing to note is that both differentials require GL-5 rated hypoid gear oils. As long as you use such, you'll be fine. Synthetics are not required, but they are recommended for longer service life. Friction modifier is not required nor needed in the front diff, but it won't hurt it at all. Friction modifier is not required in the rear diff, but it is suggested to keep the clutch chatter at bay. (Yes, the G-80 Locker has a small actuation clutch in it!).

The "easiest" option for most people is to simply get 6 quart bottles of a synthetic GL-5 oil with the FM already mixed in it, and put 2 in the front and 4 in the back, and be done with it.

X2 on Dnewton3 post. The current mobil 1 is labeled 75W-90 LS. The older bottles, I have seen a few on the shelves, do not have the LS in the title. However, read the bottle info/ingredients and they are exactly the same with FM's and LS additives. Mobil 1Delvac 75w-90 does not have the LS additives, but I have rarely seen it anywhere other than at oil wholesalers. I run Mobil 1 75W-90 LS in my G80 rear end.

One thing about up front, I have read here in multiple posts that if you do "upgrade" to synthetic 75W-90 in front diff, that there is a plastic, threaded plug or sensor that needs to be changed from one type to another. It needs to be changed to the white one, if it is not already white. Again, can't remember specifics, but I am sure someone will jump in and fill in the blanks for me.

One thing about up front, I have read here in multiple posts that if you do "upgrade" to synthetic 75W-90 in front diff, that there is a plastic, threaded plug or sensor that needs to be changed from one type to another. It needs to be changed to the white one, if it is not already white. Again, can't remember specifics, but I am sure someone will jump in and fill in the blanks for me.[/quote]

Haven't heard that one before...I'll definitely be checking back here again...I wanna know cause I haven't been too long ago changing mine...:eek:

I wonder if Heath knows any information on this subject...

One thing about up front, I have read here in multiple posts that if you do "upgrade" to synthetic 75W-90 in front diff, that there is a plastic, threaded plug or sensor that needs to be changed from one type to another. It needs to be changed to the white one, if it is not already white. Again, can't remember specifics, but I am sure someone will jump in and fill in the blanks for me.

Haven't heard that one before...I'll definitely be checking back here again...I wanna know cause I haven't been too long ago changing mine...:eek:

I wonder if Heath knows any information on this subject...[/quote]
Remembered it has something to do with the vent for the front diff and that it should be white, there are two types. Sorry I can't find the threads to link to, but I know I saw it here somewhere. I am sure that someone will eventually fill in the blanks or correct me if I am mistaken.

Found it, only applies to 2003 and earlier, your late model years should be okay. Here is link:
http://www.dieselplace.com/forum/showthread.php?t=232580&highlight=front+differential+vent

You are correct about the vent cap; it's a compatibility issue with the cap/seal and the synthetic fluid used. They changed the cap color to help visually identify the change for a quick verification of the "upgraded" compatibility.

I didn't mention it in response to the OP because he has an '07, so it's not an issue. It is good to include though, because some people with older trucks may not be aware.

You don't need the additives because it calls for synthetic. I used Amsoil severe gear synthetic in the rear and Royal Purple in the front.So far so good.

thanks guys....glad to hear that I'm in good shape with my NBS...:)

Thanks everyone. I guess I will put mobil 75w90 in front and rear. I have 07 classic so I guess I should have the proper vent cap in the front??

Yup.

Assuming mostly highway use and pulling under 10,000 lbs. about one weekend a month or less, what is the service interval for the two diffs?

Check owner's manual, but I wouldn't worry about it for drain/fill for 50k at least with Mobil 1. I periodically crawl under and check levels tomake sure I'm not losing fluid, but unless you are under severe duty, you shouldn't have to touch these very frequently.

Check owner's manual, but I wouldn't worry about it for drain/fill for 50k at least with Mobil 1. I periodically crawl under and check levels tomake sure I'm not losing fluid, but unless you are under severe duty, you shouldn't have to touch these very frequently.

Thanks, John.