Can anyone recommend a good synthetic lube for the differential and transfer case (2000 4Runner auto)? Also, what are the fill levels/quantity?
My mechanic owes me a favor so he said he could put it up on his lift and do it for free. I'll supply the fluids. He said he would also flush them out with brake fluid before refilling. Is this safe to do??? I've never heard of this.

The book calls for:
The front 75w-90
rear 90
Transfer 75w-90

I can't seem to find straight 90 gear oil. Is this a typo?
I think i'll just use 75w-90 in everthing (that's what the deal says they do).

I used redline synthetics in my differentials, transmission (manual), and transfer case. You can use MT90 for the tranny and transfer assuming you have a manual. For the front diff I used 75w-90ns and for the rear I used 75w-90. I don't have my manual but the service quantities are listed in the back of it. I believe it was a little over 2qts for both front and rear diffs and 1.5qts in the transfer. The tranny was somewhere around 4qts but I spilled quite a bit while filling.

Check out 4crawler for more info

As for a brake fluid flush, I have never heard of this before but I would not recommend this. He may have meant brake cleaner but I doubt he would use this on internals. Just ask him to drain and fill.

Amsoil. Fill till you see some coming out.