Don't hit your head too hard Jack
IIRC Ford used RTV as the sealant for the covers from the factory, I know older GMs did.
RTV is also good and sometimes required for certain intakes, I know my GMs and Dodge require RTV. The trick is not to get it in your oil pan unless you want no oil pressure.

 

I used the Felpro gasket not too long ago......

 

You will know if it leaks, mine did from day 1 when I installed it a couple of years ago.

 

When it comes to the rear ends on these trucks, I don't use mobil 1 or anything with the friction modifier built it. Too much of the modifier and when you need that rear end to lock up, you'll end up with a one wheel peel.

To that end, I use 85w140 (yes I know it calls for 75w140) and add friction modifier until it stops chattering. I did this on my '05 when the pinion seal started to give and never had a problem with one wheel peel, both wheels grabbed when I needed them to.

 

Now he tells me

Just Kidding. Hopefully I can keep the big girl out of the mud so I don't have to fight with a peg leg rear end. I did have to mix a 1/2 a quart of 75W-90 Mobil 1 with the 75W-140. I ran out and used the leftover from my Dodge's rear axle service.

 

Is it a full synthetic oil?
Did it slow or stop the leak?

 

Neither, I replaced the pinion seal and did the fluid at the same time.

 

Correct answer.

 

The limited slips aren't that good. My truck had 112K on it when I got it and it barely worked at all. I rebuilt it and shimmed the clutches tighter so it would work better, but I shimmed it to tight and turned it into a Spool. After driving it for 6 months like that I yanked it out and put a Detroit locker in. The Detroit works great, it's something I should have done years ago.

 

I towed the camper yesterday for almost 150 miles to the shop and back. The rear end did not chatter or grab once. That either means to me the fluid is doing its job or the LS disks are shot.

 

Try putting one rear tire on snow or ice and see what happens. If yo udo the one wheel spin the LS is not working.
How ever if you take off like a rocket then it's working.

 

My luck I would get stuck. Been stuck in the mud with the Ex 2 times this year, I can almost bet it is not working.

 

The other way to tell is with the test blocks.
But you really don't want to pull the carrier out just to stick
the test blocks in and check the break-free torque value.

 

I replaced the rear differential fluid with Valvoline Synpower Synthetic in 2014. I remember there was some discussion about whether the fluid would work properly in my 2003 Ford limited slip differential even though it said on the bottle that it was for limited slip differentials. I called Valvoline and they told me to add one of the small bottles of Ford limited slip fluid to their product. I did and everything has been great since then. I used RTV sealer as called for in the Ford shop manual and I have no leaks. I have always been pleased with Valvoline technical support.

 

For what it's worth, I have an F-150, with the 8.8 limited slip. I use Lucas Oil, and add the friction modifier. No issues.