BEWARE OF PTU and REAR DIFF OIL LEVEL from the factory !

[BouncinBrandon]

I have a 21 OB with a build date of Oct 21. Turns out the PTU in mine has a drain plug, other build dates might not have the drain plug. Even without the drain both the PTU and RDU are absolutely serviceable although it's a pain to extract the oil from the PTU. I used a syringe with a clear flexible hose about 24" long. I was able to get 24 ounces back into it before it started dripping from the fill hole. That's 24 out of 32 ounces (1-quart). The RDU was much easier with the drain and fill plugs very accessable. The RDU holds roughly the same as the PTU, about 24-25 ounces out of 32 ounces (1-qrt). I used Amsoil Severe Duty 75W-90 Gear Oil for both. The piece of mind having both units filled properly and by having Amsoil in them is just one less thing to worry about. I use Amsoil SS in the motor and trans as well. I do 30K changes on the RDU, PTU and Trans.

The trans oil change is easy as well, (drain and fill). I can get 4 quarts in using that method. The important thing is to drain the trans, install the plug and refill using 3 quarts with the engine off. Once you fill with three qrts start the engine and let it fully warm up in park, that takes 10-15 minutes. When hot leave it in park and fill with the remaining quart until it starts dripping from the leveling hole. I put in 5 quarts to get a better mix ratio with the old fluid, kind of a sacrificial quart as it will drain out. After the fluid stops dripping plug the leveling hole and you are done. The fill access is directly under the air box support point at the corner of the battery. Remove the airbox and the support grommet and the trans vent is right there, direcly below. Pull the vent cap off and that's it. The hole where the grommet is also works as support for a long neck funnel. Really good engineering on Fords part to make the fill point so accessable.
Using Amsoil is a personal choice, there are several other top shelf products that are very good. Using Motorcraft oils will do the job as well. Good luck all.

What research did you do when deciding to put 75W-90 in the rear diff? Upon some research and calling Amsoil Technical Support, they don't seem to actually recommend any products for the rear diff due to none of their products meeting the specific specifications. How has your experience in using the 75W-90 in your rear diff been?

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