Hi Steve,
If you mean that mastic that oozes in and out of, say, from behind the awning rail, due to daily temperature fluctuations - then I'd leave it be - the mastic sort of breathes in and out
If there's a big glob of butyl mastic on a panel, then Ray's recommendation of white spirit is the one to follow.
If it's the type that sets and doesn't soften in white spirit, then you can buy a dedicated cleaner - Innotec Seal and Bond remover - but check for compatibility first.
Robert
Robert