No issues with LPG in my 3.5EFi.
Never used flashlube.
Do run on petrol so it never sits in the tank for long & always on supermarket unleaded, rarely put more than £20 in at a time unless I'm doing long distance & LPG might be hard to find & I fill up with LPG & run on that until I have to switch to petrol.
Did a top end refurb a couple of years ago.
Bores fine.
A quick hand operated tool with fine paste very quickly showed the seats & valves were in good order.
TBH that was the one thing I was curious about but having run old engines on unleaded that were originally run on leaded & heard all the horror stories about rapid valve recession I do sometimes wonder if a lot of people just jump on the bandwagon with no experience of their own. My 1950 BSA 650 - cast iron head - did 30k miles after a full rebuild & when I had a look there was no wear to the valves & seats.