300k mile old diff fluid

[bradhays]

I have a '11 Crown Vic with 300k miles on it and my rear diff fluid looks really grimy. I've been told to change it out pronto.

Been watching some utube vids on how to change this fluid out but after a visit to the local Advance store I've got some questions. Up til now, forums on the net have led me to believe that all former police cars (which is what mine is) need 75w-140 fluid with friction modifier, but after giving them my VIN at Advance the computer comes back with a fluid suggestion of 80w-90.

The diff tag reads VOQ(?)3F SYN
3 (1 or L) 27 8810J08

I believe that shows a 3 (L or 1)? 27 which indicates a 3.27 gear ratio, and then an 88 after, which indicates the 8.8 axle. Not sure what that SYN means, unless it means synchronous?

This car is driven VERY sparingly, long distances at 60-65mph but never hard, and operates in a climate that can get up to 95F in the summer.

When the rear is jacked up and one rear wheel is spun, the other moves in the opposite direction.

So my first question is... what weight fluid should I use? Also, I need confirmation if I should use a friction modifier.

Next is how to go about changing the oil out. I'm being lazy here and after watching a vid on how you can extract 80% of the diff fluid out the fill hole by using a flexible hose I'm thinking about just doing it that way. But if you guys think it's necessary to get all that old fluid out I'll take plate off and do it the regular way.

And lastly, if I take the plate off, should I spray all the gears down with brake cleaner to get all the old fluid out, or just let it drain for a while and call it good?

 

[tixer]

Given that yours is a former police car, you can pretty safely assume that it does have the limited slip differential, and will need the friction modifier. The nice thing is that adding the modifier to an "open" diff won't really hurt anything. it just costs a little more.

If access is easy, the "right" fix is to pop the diff cover and drain all fluid. replace (with a new gasket, if there was one there before) and refill with new product.

On my (dodge) pickup, that particular job is a breeze. (on the rear, at least.) I've never done a crown vic.

If access looks tough, or you're worried about leaks, I don't think I'd be above doing the "siphon and refill" thing.

I think I would do the friction modifier though, just to be safe. Particularly if you've ever done a burnout, and found two stripes of rubber instead of one..

 

[driller]

Should be a limited-slip 3.27, so you want the 75w-140 plus friction modifier. If you use Amsoil, the friction modifier is already in the fluid.