Larry Dunn wrote in comp.unix.solaris:
/var/adm/sulog is opened, appended to, and closed by su whenever
someone tries to use it. There's no magic involved in cleaning it up,
you can just rotate it like you would any other log. It's not done
through syslog or anything else special, so you don't need to worry
about sending SIGHUPs around. There's nothing wrong with "cat
/dev/null > /var/adm/sulog"
There's no way to change what gets put in sulog without hacking su.
Either su uses it (the default) or it does not (comment out the SULOG
line in /etc/default/su if this is the behavior you want).