About to change the rear diff fluid..

[Fredrickson]

I don't know when/if the the fluid has been changed in the limited-slip rear diff on my 94 E-350 w /250K miles.

Aside from metal 'slivers', are there any tell-tale signs of trouble I should look for?

On the refill.. Is there any truth to synthetic causing leaks due to cleaning out old sludge at the seals?

I don't tow anything.. it is just a heavy whale of a vehicle.
so is there any benefit to using 75w-140 over 75w-90.. and what about using 80w90?

 

[Cubey]

I don't know when/if the the fluid has been changed in the limited-slip rear diff on my 94 E-350 w /250K miles.

Aside from metal 'slivers', are there any tell-tale signs of trouble I should look for?

On the refill.. Is there any truth to synthetic causing leaks due to cleaning out old sludge at the seals?

I don't tow anything.. it is just a heavy whale of a vehicle.
so is there any benefit to using 75w-140 over 75w-90.. and what about using 80w90?

See what the oil that comes out of it looks like first before you decide.

My 85 E350 RV had what looked like the original oil as of last summer. I put in cheap SuperTech 80w90 from Walmart ($15/gal, takes about 1gal to refill a Dana 70 open diff) with a reusable LubeLocker gasket that requires no RTV.

I changed it out again within the last month after about 2,500 miles, give or take. The first bunch of the oil came out not bad but the second half (from the bottom I guess) after the cover was pulled away was dark and horrible. It needs several more oil changes to get it cleaned out properly, which is why I went with the reusable gasket. It pays for itself, vs RTV and paper gaskets. No scraping either, so what takes the most time is using a hand pump to refill the differential from the gallon jug.

This time I put in SuperTech conventional 75w140 since the general consensus is that 140 is better for heavy duty use. It's the same $15/gal as 80w90. Not worth wasting money on synthetic if your differential is nasty and needs to be changed many times in a short amount of miles.

 

[Greenie]

I use Ford Friction Modifier to the fresh gear oil- one or two small bottles - I can't recall.