I still use 1 or 2 steel balls in my diffs (12 and occasionally 6 o'clock) with the remainder being tungsten carbide... ages ago I recall reading (can't find it now) that this was a good way to stop the plates fouling.
I've always done this, my plates don't foul but is it necessary?
NB - the steel balls remain round - I suspect this is because the load on them is negligible.
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