something you may or may not know, and is a useful bit of info I found out recently.
early XE's 88-91 BE shells were lead copper
91-93 were lead/copper but 1mm narrower
93 onwards were cheapo aluminium/tin
whenever you replace em, go for the wider lead/copper, no matter what the age of your engine - they will fit ok.
the late bottom end I have had flavour #3 - and even though they were not original, showed some alarming scoring - the old ones (80k old) out my current engine were perfect in comparison - no wear whatsoever.
don't know if this goes for the main shells too - quite possibly.