shimano has a known problem with their seals leaking in cold weather, for me it seems to happen in very cold weather 10-15 degrees.
Shimano has replaced 3 xtr calipers for me so far. the warranty for xtr is 3 years, 2 for xt, theyve allways included new pads as well since the old ones are oily, make sure you sand and clean the rotor

supposedly they have fixed the problem but I havent had mine that cold yet. should know this season. my older xt (05?) also have started leaking. so since theyre on a pugsley Im replacing them with BB7s

That makes sense. I thought something had gone wrong with my front brake (pre 06 deore) when it pissed oil all over the floor through the piston. Hasn’t affected the rear brake of the same vintage though. Will a rebleed fix the issue, or do I need to think about looking at a new front brake….?

I posted about this a while back as my LX M585 were playing up horribly and did the same last winter (although they survived and worked well for the rest of the year). I’m not sure about the oil thickening, the general concensus seemed to be that the rubber in the seals hardedens up. With each succesive pull on the levers the travel reduced until there was virtually none and the pad were in contact with the rotors (not enough to lock the brake but more than enough to be really annoying). When my brakes need replacing I’d be interested to know if there are any Shimano models that don’t suffer this.