Pjur Original. It's what we used last night, my dirty little slut.
http://www.pjur.com/products/favourites/original/
Unlike water-based lubes (KY, Astroglide, Sliquid), silicone-based lubes (Pjur, Wet, ID and Moist) are ideally suited for backdoor play. They don't become sticky and dry out like the water-based lubes. They are much thinner and feel silky compared to the relatively thick water-based lubes.
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