The stock drain plug has a magnet attached to it.

 

75W90 is too thin imo

https://www.s2ki.com/forums/s2000-un...018-a-1188044/

 

...but would not cause that sort of damage.

Honda dealer maintenance has killed more than few diffs. Maybe this one too.

If you are going to run slicks, and your diff is already toast, its a no brainer to go with puddymod.

 

Then why do they sell aftermarket magnetic ones...? I suppose its cool to have aftermarket ones available in case you strip the head of your stock one

 

Since there's nothing ferrous in the oil I assumed the "sludge" on the magnet was minute steel or iron particles worn off the gears. Not so?

-- Chuck

 

Yes I think that is correct Chuck.
Why aftermarket magnetic ones? Well I heard blue ones gives you at least 5hp
Seriously though I'm not sure I've seen ones specifically made for the diff, I guess you could always fit a stronger magnet.

 

Lol.

Stronger magnet may be the only difference then, apart from colour.