Formal:
* The student's retentive memory allowed him to recall complex formulas with ease.
* The new software boasts a highly retentive data storage system.
* The artist's mind was incredibly retentive, storing years of observations and inspiration.
Informal:
* My grandma has a retentive memory – she remembers every birthday and anniversary!
* That new phone has a really retentive battery – it lasts for days!
* I wish I had a more retentive memory for all the names I'm supposed to remember.
Figurative:
* The soil in this garden is incredibly retentive, holding onto moisture for weeks.
* The company's new marketing campaign is designed to be highly retentive, keeping customers engaged for longer.
Note: "Retentive" can also be used in a negative context, implying a person or thing is unwilling to let go of something, but this is less common.