From my reading, I've surmised that the "balk ring" syncros in a 901 need friction to operate properly. They are the band that is responsible for making sure that complimenrary gears are rotating at the same speed, therefore when engaged, you get a smooth shift .... no grinding.
Synthetics (I've read) are too slippery for this type of syncro in a 901 to operate properly. The brass slider has little to do with proper engagement of gears, as far as I can tell from looking at the guts of my 901 gearbox.
I want to try the Swepco 201 and see what all of the fuss is about.
BTW, my engine parts should be ready to pick up this weekend
Sandeep