Yeah it does stain white stuff.
But. It's VERY good. Have been in Oz since last July and used P20 all through summer here. 40c in the shade, and 50+ in direct sun. Wore p20 while out on the bike, swimming in the sea, and just pootling about. Didn't get burnt once. It's a little pricey, but hey, some things you don't really want to get the cheapest.
I bought a little UV monitor and even on the not-so-warm, not-so-sunny days UV levels can be really high here. Slip slop slap.
BTW one of the most common skin cancers here appear on ears – worth remembering when you're covering up.