I prefer silicone grease, more "slimy" so easier to make sure it gets into the crevices of the grooves without stopping the seals going home. If the calipers have seen a lot miles without being stripped down there a good change the pistons are corroded. Stainless are available although a bit pricey. Get stainless bleed nipples as well while you're at it, and stainless pins if you can find them.
These guys are good for parts ..
https://www.powerhouse.uk/index.php
But if you just want seals it might be worth going OEM