I am looking to change my differential fluid and wondered what brand you guys prefer? I've heard that Red Line is good but some differentials don't like it, never heard anything bad about Royal Purple or Mobil 1. I am defenitly wanting to go with a synthetic.

- Chris

 

I like Redline for limited-slip units. Any of them will work fine for an open diff. As with engines, dont do it if you notice any seepage. On a GM axle, synthetic fluid (Valvoline) immediately found a weak seal and started to leak.

Jim

 

Mobil 1 75w-140, with 4 oz of friction modifier if you have a limited slip.

 

Don't forget the Ford modifier.

 

Jim, I know this is a stupid question but did you use a gasket on that GM differential?

I have just the basic truck, manual 4.2L V6 4x2. I am certain that I have an open differential. If I have an open differential due I need to add in the modifier or is that just for all differentials?

 

Different Jim, On a GM dif, you should check to see if the differential has some oiling holes on each side of the diff. They are about 1/2 round and are in the mounting surface of the differential and in line with the axles.

Looking for the holes is the best way to see if you do need a special gasket with matching holes. Another way to check is that there are dimples/bulges at the edge of the differential cover that are in line with the axles, one on each side. I think the dimples are about 1/2 inch long, narrow rectangular or L shaped.

Not all GM diffs have them but some do. More trucks than cars had them. The purpose of the holes is to supply oil directly to the bearings at the end of the axles. GM quit using the gaskets with the holes to allow the oil to flow, sometime in the early 90s. OEM gaskets and a lot of aftermarket do not have these holes. They figured by the time the axle bearings went it would be out of production, at least that is what some of the guys on a chebby site claimed(you will see a lot of chebbies for sale saying they have been Felpro'd, the gasket is what they mean.

The two brands of gaskets I used on my 96 Chebby SS were Felrpo and something like Red or RXXXsomething a big name brand I hear. The holes in the gasket were narrow rectangles matching the dimples in the diff cover.

The axles still get oil from splashing, but many of the chebby guys felt that the extra oiling as originally designed in by GM was important for long life. Worst case you can punch or cut holes in your gasket if necessary.

Your diff MAY NOT have these oiling holes so what I say may not apply.

Open diffs don't need the limited slip additive.

Good Luck,

Jim Henderson

 

No modifier needed. That is only if you have a LS rear...

Also...if you are going with Royal Purple or Mobil 1, then you do not need to add a modifier anyway. However, if you go with Motorcraft...then a modifier is needed.

http://www.mobil1.com/USA-English/Mo...75W-140LS.aspx

http://www.royalpurple.com/prodsa/mgeara.html

Hope this helps...

biz

 

Thanks guys!

 

Somehow I thought he said he had a LSD, sorry.

 

Don't be sorry, your trying to help!

- Chris

 

Would overfilling the rear differential with fluid hurt anything. Did that yesterday on my 86 Ranger. Filled it till fluid leaked out the fill hole.

 

Most of us fill until fluid dribbles out of the hole and that is fine. But various manuals I have read say you should stick your pinkie in there and the oil should be about 1/4 to 1/2 inch down from the hole. So yes we are overfilling, but it isn't enough to worry about. BTW I have also seen other magazine articles and maybe even a manual or two that said fill til you dribble, so that is counter to the latest manuals I have read.

If you filled it up so oil was gushing out or you had the truck tilted on a hill side and filled it up then maybe you would have some foaming or some might spit out the vents. Otherwise I wouldn't worry about it.

Good Luck,

Jim Henderson