Some swear by it, others swear at it. Hylomar is a non-hardening sealant, which means it won't clog your oil passages if you get a small amount in the engine. But it also means it doesnt' seal as well as some other sealants. (Try very hard not to get any in your engine, regardless.)

Apply just the thinnest coat possible with the tip of your finger, being especially careful around the oil galleries above the top cylinder studs. Avoid at all costs having excess sealant squeezed into the oil gallery opening when the cylinder is snugged down.

If you can't find Hylomar itself (try Ebay), you can buy it as Wurth DP300 from your friendly BMW dealer (ka-ching!)