anti-seize or?
 

The bolts that hold my front calipers in place (02 Boxster S) and the screw holding the brake line bracket to the strut tower have a blue substance on the threads. Based on mileage, I'm guessing this is the first time they've been removed. So, is this an anti-seize compound or a thread locker? Should I be applying this substance when I reassemble? What do you guys think? I will torque them down to factory specs.

If it were me, I would clean those bolt threads good and apply the blue thread lock and torque.
You will feel better knowing they won't back out......it's the ticket!

ABSOLUTELY use anti seize on the calipers, NOT thread locker. Do a search for the number of people that have had to either have time-serts or Helicoil installations on the calipers, or had to replace the calipers when the alloy threads came out attached to the bolts. Anytime you are in doubt, reach for the anti seize over thread locker; there are relatively few fasteners on these cars that need thread locker.

+1 Agree 100%

Yeah not sure I’ve ever used thread locker on any caliper bolt. Anti-seize for sure.

Believe Rennline sells a stud conversion kit to mitigate just this problem......I would be interested in whether they spec thread locker....:D

Thats a good idea if you track your car often. Their kit comes with thread locker.

Not sure I've ever seen a German car that didn't arrive from factory with threadlocker on brake caliper mount screws. Mercedes all do, as did your 986.

This is the reason the official service procedures call for replacement of these screws rather than reuse ... so you get a screw with preapplied threadlocker. Think maybe the Porsche 986 versions may stretch as well ... so again, replace, don't reuse.