Chances are it's still on the original seals and if pistons are sticking it's pretty certain the seals are buggered. Unless you're pressed for time it's worth doing the job properly.
Use a clamp to hold the less-jammed piston while you use the brake pedal to force the stuck one out.
You might want to buy a piston removal tool to wriggle the other one out.
Alternatively you can split the calliper which does make cleaning easier, though that's another seal to buy.
Strip out the old seals and replace with new (OEM).
The pistons are heavily chromed and often clean up OK. While there may be some pitting, so long as it is outside the area that contacts the seals you won't need to replace.
Duck oil, WD40, GT85 whatever your favourite water displacement fluid is… they're all basically the same stuff i.e petroleum distillates. Stick to brake cleaner or isopropanol and red rubber grease.
