300k mile old diff fluid

[bradleyheathhays]

I have a '11 Crown Vic with 300k miles on it and my rear diff fluid looks like this...

 

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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.

 

 

Here's the diff tag...
 

{pic}

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, 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?

[bradleyheathhays]

Couple wrap up questions...

What advantage is there to using the more expensive Mobil 1 synthetic as opposed to a cheaper Valvoline full syn... $20 a bottle vs. $13, times 3.

Also, I've been given advice to replace the clutches and 'steels?' in the diff as mine are probably worn out. What advantage in performance would I gain by doing this and is it expensive/ worth it?

 

[Clownking]

My opinion. If there's not a lot of metal in the fluid. I'd drain what I could and fill it back up. If there's metal in the fluid or excess noise then I'd take the pan off and see what's failing