Typically, one standard 14oz tube is enough for a fill. Two is safe, and gives enough for future top off, if needed.
I know diesel has been recommended for flush, and it will certainly work and do the job quickly. My preference when not in a hurry is just to use a good brand 75W90, which won't stay in long under normal circumstances, but will clean up the old stuff if you fill and drive it for awhile. So long as the gear oil and selected grease are compatible, all is good. Just keep a drip pan under the gearbox!
