Unguent of Timelessness says it can be applied to anything that was once living (which would include bodies); it also says it can't be washed off but can be removed magically by dispel magic. If it's put on a deceased character and that character is brought back to life, what is the result? Is it dispelled in the process? Does it become inert while the character is living but resumes action if they die again? Or does it continue functioning, slowing their aging to one day per year and giving a +1 on saving throws?
This is not an actual issue for me yet, but a party I'm GM'ing does have a jar of the stuff and they could die, so...
Just say it's dispelled when they come back to like.
By the way, that's the item my character is going to use for his clone bodies.