That's quite normal to see a magnetic paste on the magnet after a rebuild. I've fitted a magnet in the oil tank drain plug on my 16H which works well. With a newly built engine, I like to changed the oil after about 50 miles. This gets rid of any particles of metal from the initial bedding in, then 100-200 mile, 500 mile, 1000 mile. That works for me.
Fitting a magnetic sump filter is a good idea.
I changed oil after the first 500km, after that I change once a year.
Fitting a spun oil filter to a ball bearing engine is a bit of a overkill.
And it does look rather out of place although that's a matter of taste I suppose.
However, fitting a spun on filter to a plain bearing engine is a good idea, as those type of engines benefits from really clean oil
Babbit bushes can take on some dirt as it will get embedded in to the babbit, but they also benefit from a external filter.
I have to add that this first 500Km was with a completely clean & fresh engine, no traces of old oil left in it.
+ a cast alloy sumpplate with magnetic plug .(I make those for the AOMCC)
