No problem mixing at all.. Your right about the purity never being 100% with the drain and fill method, plus there will be waste..

In the drain plug thread I did detail a procedure, and linked to someone else who did a fluid exchange.. Basically just remove a tranny cooler line up at the heat exchanger, start the motor and feed fresh fluid in through dipstick, and purge the old fluid out through the now disconnected cooler line.. This is the only way to get pure fluid into the tranny..
.. After a couple pan drop cycles you should be safe to do this..

 

Is this what you were planning to do with the Castrol? Couple of drop and drains, then replace with Amsoil? I know you mentioned it in that thread...

 

As far as gear oil I use royal purple 75w140 for posi and added extra generic friction modier but any synthetic will do the job. My manual says use synthetic 90 grade or use the 140 if you do alot of towing. Change at 50k miles.
For the tranny I dropped the pan at 125k and refilled with valvoline merc V and a qt of Lucas trans fix.
Bout time to change them both again with 172, 000 miles but cant complain.

 

Yes that was my plan.. But since amsoil ATF is harder for me to get at a decent price, I'm sticking with castrol transmax.. I've been dumping the fluid every 15,000 - 30,000 miles anyways, so I don't need a 1 and done fluid.. I've done 3 flushes so I know I'm predominantly castrol fluid..

 

I used Royal Purple when the diff started to weep oil. Even though Royal Purple claims no friction modifier is needed and it anyway. As I got the rear end shutter after a couple k miles. Added the Ford modifier and the shutter went away

 

Hmm I just noticed mine is leaking now too. Maybe the PAO basestocks are too thin for those seals..
Did it stop?

 

Royal purple was the only "correct weight" available when I shopped, and I added the MC friction modifier even though the RP claimed it was not needed. 30K miles later, no issues...

 

Ford used a sealant to seal the dif cover. The leak was small like a oil residue, no drops
I redid it because a previous F150 I had had a small pinion seal leak. That ended up killing the wheel bearing, half shaft, wheel cylinder, and drum.

 

I changed my seals and wheel bearings today. The pinion seal was shot to hell and one axle seal was seeping slightly. I cant blame the fluid. 172k is alot of miles
filled it back up with RP 75w140 and extra friction mod. Good for another 170k I hopem

 

Good for about 30K miles / 3 years, in reality.

FM absorbs and retains water ( hydrophyllic).

After that time / mileage you begin to risk corrosion. Diffs are not sealed, they are vented.

Part of the maintenance regime, IMHO. Fluid is cheap relative to the cost of a diff rebuild.

Funny - folks will dump perfectly good engine oil after only 3000 miles, yet leave other fluids in for 100K+.

Don't make no sense to ol' Bubba