I am in the process of checking my oil level on my rear differential and not sure what type and weight I should use to top it off (if needed).
I have read everything from 75W-85 Synthetic to 75W-140 Synthetic. I want to match and use what OE came with .. 2018 Ford XLT 5.0 4WD.
Can someone please advise?

 

What is your mileage?
I would not expect any top off required but pretty much everyone should change the fluid if you are over 30k miles. You'll see, it will be dirty. I used a Lubelocker gasket for reassembly - no RTV needed and it is reusable.

The owners manual specifies
Rear axle fluid:
Motorcraft® SAE 75W-85 Premium Synthetic Hypoid Gear Lubricant
XY-75W85-QL WSS-M2C942-A

However, 75W-85 is super expensive and pretty rare. I used synthetic 75W-90, as well as I think many other members here. You can also get Valvoline synthetic 75W-90 in the Flexfill bag. Way easier to squeeze in than a bottle. I found it locally at Napa.
I also read about this and I think the change from 140 to 90 to 85 is efficiency/mpg driven because a few years ago the "same" rear end spec'd 140.

Here is a copy of your owners manual for future reference where I looked up the fluid spec.
https://cdn.dealereprocess.org/cdn/s.../2018-f150.pdf

 

I run 75w140 in my 19. If you want to stick close to spec, just use 75w90.

 

40,000 and I pull a lightweight 4500lb travel trailer a few times a year (if that matters). I read where a change is required @ 150,000 mi.

 

150k is IMO far too long for even a truck that never tows. Lifetime fluid... yes if that is the expected lifespan of the assembly! Especially when there is no filter. It is just bathing in all of the metal debris. And the fluid will nearly be paste by that point.

I helped a friend change the fluid on a Tacoma with 30k miles and it was dark, dark gray. My rear diff change at 60k was also dark gray. I checked it again at 70k and it still looked like honey.

At 40k I would suggest a first break-in change now and then change at your convince thereafter, maybe every 30-60k miles as you get a feel for how dirty it is over time.
It's only 3.3qt or about $30-45 in fluid. For 75W-90 Synthetic, Mobil1 is generally cheapest under $10 but I do like the Valvoline in a bag at $15.

With a reusable Lubelocker gasket it really isn't too much work other than smelly gear oil.

Also don't forget about the often neglected front diff, especially if you have a 4A truck.

 

Just curious, I have seen others suck fluid out of things, and replace. Transmission comes to mind. Is sucking out the fluid in diffs and replacing with oem a valid option? I'm sure it wouldn't be as good as a complete drain, but better than nothing.

 

Sure, there's nothing wrong with it. Doesn't get quite as clean as a full drain and wipe down, but unless you have major issues with your diff, doesn't matter that much. That's how most dealers and shops do it now anyway.

 

Yes, I used a 5L OEMTOOLS 24389 fluid extractor for a trans fluid exchange. I also think that will be easier for a fluid+filter to first extract the fluid before dropping the pan and plan to do so. It goes very fast with the low viscosity.

For the front diff the easiest option is a fluid extractor so that is what I used. The cover can not be removed without significant work (the steering rack is in the way). Some users drill and tap for a drain plug in the bottom but I haven't gotten there yet.

For the rear diff I usually just remove the cover so I can also wipe it out. I also had more trouble inserting the fluid extractor hose in the rear.

I wish Ford would add drain plugs to both diffs like Toyota (and Subaru, and Honda, and ...)

 

75w85 is the Oem fluid spec. It’s somewhere between 30-35 dollars a quart at a dealer. A 2018 5.0 should have the the super 8.8 which doesn’t have a gasket on the cover. You have to seal it with rtv. Holds about 2.75 qts.

If you want to change it, I’d use any 75w140.

 

Originally the earlier year 9.75" came with RTV and then switched to a gasket from what I read. I am not sure if only earlier or all super 8.8 also use RTV.
But you can switch to a reusable Lubelocker gasket on both diffs. Far easier for future changes.

Edit: As pointed out below LubeLocker does not offer a Super 8.8 gasket.