There is no leaded street gas in the USA, all years of the XS650 run quite happily without it. The one year only statement makes the claim very suspicious. There is nothing all that odd about the 72 engine, it is identical to the 73
The valves used are the same as all years 1970 to 1983
IMHO Use unleaded and don't worry.
At last weekends XS650 rally there were a 71 and 72 owned by a father son team, both high miles, regularly ridden bikes. They were drinking unleaded just like the rest of us.