Check this out corrosion after 4 days

[Alex Mckenzie]

Hi all
I rebuild my 020 gearbox last week but looks like I used the wrong input seal so had to pull it out again, anyway I pulled the 5th gear cover off and this is what I found.

Corrosion of the gear. Alarmed at this I dug deeper and found
More corrosion, all the gears have been affected and to a lesser degree other parts.
If put nulon sae 80 api GL4 in the gearbox on Tuesday, and the above is what I found today. How can this be?
I believe the is the right oil to use, am I wrong?

Any thoughts? Best recommended remedy to clean it all ?