rear diff gear oil

[TwoToneX]

I tried searching and found a bunch of helpful threads, but it seems for every thread there is, there's a new or different suggestion as to what weight oil you add to the rear differential. Does anyone have a definitive answer as to what weight gear oil you need to put in the rear diff of a '91? Syth. or not?

While we're on the subject, I tried putting in my locker about 2 years ago, resealed the diff, it began leaking again so I took it somewhere for them to do it, it started leaking again so I took it back, and yet again it's leaking again. So for the 3rd time in 2 years it's leaking, any ideas of how I can seal this b!tch up for a long time?

 

[Bronco638]

The gear lube is your call there is no "definitive" answer. Based on your avatar, I'd go synth. It's capable of taking the heat and load as well as providing extended life. It costs more but I think peace of mind is worth it.

As for the leak, where's it leaking from?

If it's at the yoke, you might have a burr that's causing the seal to wear prematurely. You might need to source a new yoke or use a speedi-sleeve to provide a new sealing surface.

If it's at the cover, the housing (or cover) might be twisted or warped so that it never seals quite properly. Try replacing the gasket with a Ford unit or check the sealing surfaces to be sure they're flat.

Hope this helps - D.

 

[410Fortune]

Looks like hes got you covered

I use Mobil One Synth Gear lube in my 8.8 with L/S.
Takes about 7-8 of the bottles, so it gets pricey

80W-90 is the alternative to that

 

[NOTAJP]

I just got done talking to Currie enterprises. They say, "Do not use synethetic!" Syn can and will find holes to leak out of. If you think you have leaks now....just see what I'm going through. I'm about to take out the synethetic because, #1 it's time and #2 I have leaks that I never had before.

I use power punch additive with my detroit locker. Works wonders for heat, noise, and ratcheting.

Cover gasket. Don't use a gasket. RTV works the best. A couple of years ago I had the same problem and replaced several gaskets. Finally gave up and sanded the surfaces really well and gouped up well with Blue RTV. Solved any issues. Also, check your diff cover. Tends to need replacement every so many years.

 

[410Fortune]

Why would it leak?

I mean I know synth will leak easier then conventional fluid, but if you have leaks, then fix them...

The cover is $23 from Ford, I agree RTV works fine after a quick hit with a sanding block.
If the pinion or wheel seals are leaking be sure to check your breather for a clog first ...

 

[Bronco638]

I would think that using a gasket would help seal any gaps due to poor fit, warped surfaces, etc. Seems like a lot of RTV might have to be used instead. But, I'm not going to argue with your experiences. Currie says not to use synth, eh? I thought Ford recommended it. ??

410: Good thought on checking the breather.