So ive searched my butt off and have come up with a conclusion after reading 100 plus posts. On my 06 4.6 4x4 3.55 LS this is what ill need.

75-140 Royal Purple fluid (no extra additive needed due to RP)
A seal
tools.

The questions are:

Do i need RTV or sealent for the gasket? If so do i just put a light coat on both sides and what kind of RTV do you recommend?

Do i fill it all the way up till it comes out the top or put exactly what the manual says to put in?

 

Not sure about the RTV on the gasket, maybe just to hold it on while you put the cover back on. idk. But you will know its full when fluid comes out the fill hole. It should be right up to that mark.

 

get some blue permatex, this is what ford uses. put it only on the axle pumpkin, not the cover. make sure to clean the axle and cover as thoroughly as possible to allow the permatex to seal properly.

also, you'll want a transfer pump to put the fluid back in w/, makes things so much easier.

mobil1 will work just as fine too, and is probably more available. it has the LS fluid as well...

 

The factory uses RTV only, no gasket material.

 

Is it going to hurt anything if i put a gasket in it? I got one from ford, and why only put RTV on one side of the gasket?

 

anybody?

 

so gasket with one side rtv'd?

 

I used to rebuild diffs for a for dealer and we never used a gasket. All we used was BLACK silicone, we put it on the COVER. let it sit for 10 min and fill er up with fluid. Never had a problem.

 

i've done 3 -f150's and i used red high temp silicone, and used 75w140 synthetic, and if you have a limited slip rear you need a bottle of the additive ford sells to get the clutch packs to engage correctly.

 

Additive not needed if you use Royal Purple.

You can use a gasket if you want to.

 

If it is the 8.8 rear end I would use a gasket.Fel Pro makes one;
Part # RDS-55341
Put a very thin layer of RTV on both sides.Also,even tho the differential fluid says it has the additive in it I would still buy a bottle of friction modifier from Ford and add it!

 

cool, thanks guys, i already have the gasket, and it is a 8.8 i believe.

 

Same here. I'm a Ford tech and have done countless rear-diffs. Make sure you clean the surfaces of the axle housing and the cover (grinder with a pad and brake clean work perfect) and put a bead of the black or gray silicone on the cover. There's no need for a gasket but, if it makes you feel better. I've always used Ford's 75-140 and fricition modifier, haven't messed with RP, but just fill it up till it's level with the fill hole.

 

So here is something that I dont think ahs been answered.. IS more better?

If I run Royal Purple, AND add the Ford Friction modifier. Is that as ggod / same as / or worse than running a basic oil with the modifier?

Thanks.

 

Adding friction modifier to your Royal Purple will not benefit you whatsoever. Will it hurt? Only your wallet. Royal Purple, Red Line, & Lucas Oil gear oils come with the friction modifier. That is one of the reasons you are paying a little extra money.

What I would suggest is picking up a Mag-Hytec differential cover. It comes with a magnetic dipstick, drain plug, cooling fins, larger capacity for more fluid, and a built in gasket. No more silicone gasket maker needed. You can now monitor what your fluid level is, see how clean it is, and clean out the metal shavings that are floating around. The extra fluid and cooling fins help keep the gears run cooler. And now you can change your differential fluid as often as you want without having to take the cover off.